``Frotter'' is a term with which not all are familiar, although at times it has been used widely. It refers to someone who gets some kind of pleasure (classically, with sexual overtones) by brushing against people, without their agreement, often to their irritation, and generally trying to make it look accidental. I find them extremely annoying when they do it to me... but sometimes the irritation is only cancelled when I have had physical contact with a friend, which is perhaps a related effect! I widen the term to cover anyone who touches someone for the toucher's pleasure without regard to what the person touched may feel about it (e.g older people prodding or patting you on the arm while talking to or at you), although strictly speaking, frottage refers to brushing against people. I've noticed some correlation between someone frotting people and between them tending to buttonhole people for the sake of not not being in conversation -- presumably both may be last-ditch attempts to ward off, or avoid acknowledging, feelings of wide non-acceptance that can lead to loneliness.
I get sufficiently annoyed (or sometimes upset) by being frotted that it seemed worth digging into the underlying reasons for this annoyance. My first, rather unclear, understanding of (at least some forms of) it was in terms of them feeling that they have a taint about them, and are rubbing it off onto someone else thus equalizing other people's taintedness with their own. But more recently it has seemed clear that it is largely a matter of objecting to being used as a tool to help someone make themselves feel good. This idea of tool-use (also mentioned in my essay on loneliness) helps to fit it into being one end of a spectrum that continues into doing things with friends (with their agreement) because of feeling left out and ends at the other end in kindly giving time to people you really can't stand because otherwise they would feel left out.
So what can one do for or about these people? I feel uncomfortable with deliberately letting them have their perverted pleasures. One plan with great appeal is to hit them extremely hard, but there are legal and ethical restraints on this. There are social restraints on pointing out loudly and publically what they are doing (although those constraints may be against my ethical integrity) and biblical advice is that first reproof should be private... that sounds quite good, taking them aside and giving them a good talking-to, which could include a warning that a repeat will bring about a loud public warning, and further repeat will bring about physical self-defence against what the law regards as (sexual) assault. Of course it will please some to know that they have at least annoyed, upset or disgusted someone, but I think I am sufficiently sharp that I can dissect underneath their smugness provide that I don't lose my temper!