You can bind someone to prevent them from hurting themselves.
And your second question, what you are asking is almost impossible to happen. Very few people know anything about magick, spells and workings, it's a very small chance to find someone at random. Most people are either xians (or another mainstream religion) or atheists, and the first ones believe magick is evil while the second think it's all bullshit. And it goes without saying that you should be careful around one who practices magick, i.e. people like Wiccans or or those who practice Traditional African diaspora religions like Voodoo or Santeria and not use any working against them without removing their protections first.