I got one a few days ago, tried declining it a couple of times from one of the web pages or another and was logged out each time. The invite lived on.
I ignored the invite for a few days, then had the idea this morning to decline it from the game client. I got an error message about an "invalid tournament" which I was able to close without further disruption. The invite disappeared. Success!
(To be safe, I should mention that I wasn't playing in a tournament at the time. I don't know if this will disrupt tournament play.)
I opened WGT on another machine in Chrome a few minutes ago and the invite was back. opened the game client and refused it again. Checked to see if closing Chrome would make the invite reappear; it did not.
Opened Maxthon...the invite was back. Closed and opened Chrome again with Maxthon still open and showing the invite...no invite there. Tried refusing the invite from a WGT web page (my profile I think) in Maxthon...success!
Summary: Refusing a CC invite in the game client seems to work. The invite might still show on other browsers or other machines, and it might be possible to refuse that invite on the web page after it has been refused in the game client elsewhere.