Then you just switch to 'grapeshot' mode. Instead of one big slug, you blast out pellets in a cone like a shotgun. Unlike a shotgun the range is infinite amd put enough velocity behind it and even a ball bearing could go straight through the hull - or at least make a mess of exposed things like sensors or windows. Even better the shot pellets are 'cold' with no emissions so the only way to detect the shot is with active sensors, which means you just waved a big sign saying "Here I am. Shoot me!"
Or even cheaper option, just bolt on a disposable missile pack or two. So the next time a ship full of kids tries to be clever, the system just fires off its missiles while locking on with the BFG.