You actually need to go to their house and destroy their PC. It's the only way I know..
With the click of a button ;)
At their user page at the bottom just above the Block option .
Click " Remove from Circle "
Generally, you warlock them.
Sorry, but you kinda walked into that one. Now, the real answer. You go to their profile, scroll down, on the list on the left side, there is an option "Remove member"