That was my best idea.
I think just deleting accounts and blocking those IPs as fast as you can is the best you can do without writing a plugin that tracks all IP numbers. People are very concerned about IP tracking, so Discourse keeps only the first and last IP numbers.
But they likely have more IP numbers than you can block one-by-one anyway. Maybe change the TL0 settings so that TL0 can do less and it’s harder to become TL1.