> sho xlate [global | local ip]
will show you the translation for a specific local or global IP address. You can then clear this particular translation with:
> clear xlate [global | local ip]
See the following URL for details.
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_62/cmdref/s.htm#1063516
You can't really block it without an ACL, but you could stop it being NAT'd (effectively stopping anything outbound from it) by removing that IP address from your NAT statement, could get messy depending on how your subnetting is set up though.
