Gun dog training
Unless ordered not to, a hunting spaniel will naturally tend to follow the line of the last bit of game it encountered.
The best way to stop it is to train the dog from an early age to ‘leave’ whatever it is you don’t want it to pick by way of a retrieve or, in this case, a scent/sight trail of a departing bird or rabbit.
What you need to do is let the dog stop to flush, give the command ‘leave’ and hold him there for a few seconds just to calm things down a little.
Now get him to hunt in the opposite direction to the flush by giving the relevant whistle/hand signal and don’t turn him again too soon.
If he should try to turn and follow, blow the stop whistle and, again, tell him to ‘leave’.
Hold him there a second or two before turning him in the direction you want with a hand signal.
Provided you are positive, clear and consistent with your commands it shouldn’t take too long to break the habit he’s fallen into.