A foul in the course of normal play is a different thing from a non-footballing piece of dicking about!So it's a caution if you've warned them? And what if you forget to warn them? I don't like the idea that just because the referee has said something, an offence is penalised more harshly...
I say 'no foul' during play, if a player then fouls, should I caution him for being unsporting and not listening?
I do think you can card for this, but I'd put it through as dissent by action. You've been told not to do something by the referee and you deliberately choose to do that thing. Surely that can only be an attempt to undermine his authority? In the book, in the sin bin if appropriate.