On closing, I agree with previous authors: enclosing circle just feels intuitively right for the word.
[img]http://lh3.ggpht.com/_y4aqSnXq6kA/TQTU5R3djDI/AAAAAAAAABw/iKTVoe3Sy9s/close.jpg[/img]
Cancelling is more difficult. I perceive the square as the house, the shelter, the strength of protection of what it on one side of the square walls from what is on the other (inside or outside).
So the counter is the symbol that breaks the walls, that makes the protection obsolete no matter how much faith one had in its strength.
[img]http://lh6.ggpht.com/_y4aqSnXq6kA/TQTU5tw368I/AAAAAAAAAB0/s9EjVAad1-w/counter.jpg[/img]
We also need not forget that as time goes by, new symbols appear and get entrenched in our minds. So if the square is the symbol of a stop - then the counter symbol is
[img]http://lh4.ggpht.com/_y4aqSnXq6kA/TQTU5akKFoI/AAAAAAAAABs/vu1tMFrrS6w/fun.jpg[/img]
play )))