A hack client out right now allows you to place a sign which the line goes over the max string buffer Minecraft can handle; however, it only works on 1.8.6- servers. People have placed signs in places many people go and when you get near them you get kicked from the sever. Update to server version 1.8.7+ to fix people from being able to place the signs. As for deleting the signs people have already placed, you can either revert to an older version of the map(s), remove all the signs in the area using worldedit (which will probably remove important signs), or get someone to make a plugin real quick to delete any sign containing the '|' character, as that is what the client uses to create the glitch.
src: https://www.spigotmc.org/threads/error-io-netty-handler-codec-encoderexception-string-too-big-was-32789-bytes-encoded-max-32767.71401/#post-789934