The cancellation button will not show up for any pending invitations until they're two weeks old. After the two week period, you should be able to cancel an invite. Here's how:
1. Go to your CafeMom inbox. (You can find it by clicking "My Messages" at the top of the page in gray)
2. On the left, click the "Invites Sent" link.
3. Click "Cancel Invite" below the invitation you would like to cancel.