Plot twist:
At first the owner of the public sandbox thought : "Oh well, kids will be kids! The problem will sort itself out by itself." So he did nothing, and tried telling the other kids to "Gang up against those two and push them away."
At first, the bigger group of kids tried. But they found that the more they tried, the more unruly kids arrived, and the less time they had to build their sandcastles and play together peacefully. They also found that trying to push the unruly kids out of the sandbox only seemed to encourage the unruly kids, which turned the sandbox from a happy place into a bit of a permanent warzone, which took up all the time the majority of the kids, and none of them could build their sandcastles or play other game with each other at all.
The owner, seeing all this, shook their head, realised doing nothing wasn't solving the problem, and took steps to help the majority of the kids play happily again.