Is it possible to cancel the troops I dispatched for attacking someones city?

Cancelling a march requires that you have researched 'Recall Order' In that APCs research thread.You then also need to purchase a recall item for 50 diamonds to use.

An extreme alternative could be to try to teleport to a new location. This may recall all APC's although I cannot confirm this as I haven't tried it. You will need to have a teleport item to use as well though.