PM them and let them know or hop on IRC. I don't know if they can cancel them or not though.
Specifically, PM sN0wBaLL, since he is the one that does the trainer cards. -FW
<img src="images/trainercards/pixeltwist.png"><img src="http://img.photobucket.com/albums/v497/insanity122/side.png">
<img src="http://tinyurl.com/2p7b96">OMG PSYBLOG!