i tried googling to find an answer but no one seems to have talked about this before i can find.
so we are fighting a guy hiding in a tar pit, hes using spring attack to fly out make an attack and fly back in. i readied a battering blast to hit him when he comes out and knock him back. i was hoping to interrupt his spring attack and trap him up top for the round. the DM said he still had enough movement to finish his attack and move back into the tar after my knock back finished pushing him away. but i was under the impression that forced movement interrupted your movement and you lost that move and since spring attack is a full round action he would lose the rest of his turn???
The GM is correct.
The way this is usually done is to trip an ememy mid-charge (or mid spring attack). This would mean a land-based enemy would become prone, which would prevent further movement (they would then need to spend a move action to stand up). In this scenario you would interrupt their full round action by denying their ability to continue it.
Bull Rushing a creature simply moves it on the battlefield, and doesn't impose any conditions. While you interrupt their turn, they would still be free to continue their movement.
It wasn't a bad idea, if they hadn't had enough movement to get back to the tar pit then you would have changed the dynamic of the fight. At the very least you did damage to them and moved in initiative to just BEFORE of the enemy's turn.