Natalya,
If the man/sex is too rough, stop and tell them. If they continue to be rough, tell them to leave - just because someone is paying you, it doesn't mean they can treat you like this. Remember this. If you're passive and ignore bad behaviour like this (and lack of manners), these clients will return and could push boundaries further.
Why are you scared of blocking someone? What do you think they will do? The answer is very little. As you say, men are quite stupid, but neither can you rely on them to read between the lines of what you're telling them or understand the signal; you need to be direct. In this instance, if you really don't want to tell them or to block them, just ignore their messages and don't interact with them.