You've raised an interesting point about the position in law if one uses force to prevent people from harming themselves. The starting point is that any deliberate use of force against a person is a criminal assault, whether or not it causes injury. The prosecution must prove that the person in question did not consent, although in some rather curious caselaw our courts have created exceptions to the defence of consent regarding sexual activity: gay SM sex bad; man branding his initials into his wife's buttocks good, the Court of Appeal finding - as recently as 1999 - that it is in the public interest that men should be able to do that kind of thing without fear of prosecution. Away from those rather unusual situations, the courts will imply consent if it's reasonable to do so, examples being contact sports and the situation where someone pushes up against you in an overcrowded train carriage. There would be a strong argument for saying that anyone suffering a violent epileptic fit may be taken to have consented to reasonable force being used to prevent that person from hurting him/herself until they return to their ordinary rational state. Once that happens, it's worth remembering that mentally competent patients have an absolute right to refuse consent to medical treatment, however rational or irrational. Finally, the law allows the use of reasonable force to prevent the commission of a criminal offence, for instance to protect oneself or a third party (someone other than the person using the force) from an assault. There's no magic about the word 'reasonable': it's what's reasonably proportionate in those particular circumstances.