SM doesn't make it easy *sigh*
For me blocking guys I have to be really fast on the button.
In the list of customers, click on his name (this alone can be frustrating, since the names shift while custies move in & out of free chat).
Next to his name there will be 3 (I think) gray icons
one is temporarily kick them
another is block them (will give a pop-up asking about emailing too...I find this step very annoying)
last, the little man, will bring up a 'Notes' box which allows you to make notes for yourself about this custie. Having notes about a customer is great for making him feel like you remember him, and builds rapport =)
If someone has been popping in & out of paid chat, and you can't block them while you're doing the show, write to model support, give them his name & what time/date (and your time zone, so they can find him) and support will block them for you.