How can I cancel an invite?

In Booking centre your invites show in dark blue. Click the invite button on one of your invites and a window will show you all details for this invite.

There's a red Cancel button at the bottom. Click that.

You can also see all your invitations for a particular event (League or friendly) by clicking on that event in the "My invitations" panel in your home screen. The popup window lists both your invitations to others as well as other people inviting you. For your invitations you can click the red bin icon to remove it.