This plugin was quickly made to prevent people from crashing other players, it stops people from placing signs that have more than ~16 - 50 characters on each line
You can use it too.

Server Sign crash quick fix Version 3
There is apparently a mod that allows you to crash a server with a sign
Recent Updates
- Updated for characters like "||| ***" Jun 21, 2015
- Fixed minor bug Jun 20, 2015