No. Never. Absolutely not.
Well, almost never.
Chances are, if the person has decided upon their course of action, that your take on the situation is not certain enough to justify infringing their rights.
However, there exist clear exceptions. A normally stable person who, on an LSD trip, wishes to jump out of a window may be restrained. One could reason that the drug has already deprived them of their capacity to make informed decisions, and preventing them from carrying out uninformed decisions is not nearly so great a crime as restraining them when they are of sound mind.
Someone stepping into the path of a speeding car may be yanked violently out of the way. Again, their decision is clearly uninformed, and the harm done by their benefactor is demonstrably insignificant next to the harm they would have suffered otherwise.
Normally, it is not your place to decide for another whether their decisions are informed, but as has just been established, there is a threshold of certainty beyond which this changes. The simplest way to consider it is: If you have to ask whether a situation is so exceptional that you are morally allowed to initiate force, it is not. The only such situations will be completely obvious.
This has two problems: A cretin may consider it totally obvious that they have to do all sorts of nasty, restrictive things for people "for their own good".1 And an overly cautious, overly analytical person may question even that which should be obvious2, creating unjustified uncertainty.
I shall deal first with tactics for the latter type of person. Instead of delineating a line of certainty, the problem becomes finding and evaluating a threshold of certainty, weighted by the consequences of your considered action versus theirs.
An exhausted and sleep-deprived friend insists on staying up working in their dorm room, but is clearly no longer being productive. Do you put them forcibly to bed?
A friend has a date with someone whom you know is a rapist, but she doesn't believe it. Do you prevent the date?
No violation of rights may even be considered a possibility unless persuasion has already been ruled out. Try to convince the person of the danger of their action. Give them the information that brought you to your conclusion. Sadly, this is not always possible. Information is not free. In the time it takes to communicate about the speeding car, the person will already have become roadkill. Your years of trust-building experience with the companion who told you about the rapist might not be communicated in any convincing way to your friend who has never met this companion. The sleep-deprived friend may not be in a sufficiently receptive state to even listen to reason.
So you have to consider whether you can act to stop someone. The system I present for weighing one's options is designed to minimize the evil done to the person under consideration, by you and by their intended actions. Though it uses a mathematical form, the variables are not measured in units. Rather, varying levels of rightness (or wrongness) are compared, which requires a sense of proportion regarding these.
Judge the evil to which the person may be subjected by their actions. Now find the average evil you expect them to suffer from the actions, by multiplying this by the probability that the evil will occur at all. (Any lesser evils should be similarly weighted and added in.) Now for the most tricky part. Take the average expected evil that you have just determined, and reduce it in proportion to the probability that you are just plain wrong, to find the average actual evil. Now compare this to the evil to which you would subject the person by violating their right to their own decision. Choose the least evil path. When weighing the first variable, take into account that people learn from getting hurt by their own mistakes.
Weighing the probability that you are wrong is a difficult task. The fact that the person disagrees with you is almost always a big strike against certainty on your part. And people are generally more certain than they ought to be in such cases. You really can't be too cautious on this one.
When weighing the problems that your considered infringement would bring about, keep in mind that not only might the person resist your attempt at prevention, their resentment of it can raise a defiance that will strengthen their inclination to go through with whatever dangerous act you were trying to prevent. And it will probably hurt relations between the two of you.
This is not air-tight, however. While it has the advantage of granting power in proportion with a person's analytical ability, self-awareness, and understanding of the situation, it has the severe disadvantage of allowing people who overestimate their own mental prowess to assume that it grants them the same power. So what I would like you to do, before considering me to have handed you a blank check on morality with an equation, is to make a few judgment calls. Have a pen and paper ready, and:
2. Let's have a big round of applause for stumbling into cognitive traps in the effort to avoid other cognitive traps.