The punch packet (I believe this is the "UDP ping" that Blockland uses) is 6 bytes in and 6 bytes out; the punch packet is literally nothing.
The server authenticates BLIDs before checking the ban list, which is a design flaw in the game. The ID should be assumed correct and matched against the ban list so that simple "attacks" like this are not possible.
I believe Blockland blocks high-rate pings but rapidly joining doesn't trigger a very fast rate.
Try to block his IP with your router or computer if you can.