And most of the times, lack of resources.
It's not as easy to deter someone with whale level SP. This is demonstrated again with the recent thousands of comment spam that the frontends had to make invisible.
It's a dedicate balance to hit what you can actually knock down. Otherwise, several things can happen.
Abuser carries on, you didn't make a dent to profit, but focusing on the big one causes the little ones to grow unchecked.
Big guy comes swinging back, lowering/hamper your growth, while still reaping themselves. Smaller abusers get a temporary (or longer) growth spurt.
etc.