[Server] Random console spam (GC overhead limit)

Hello ladies and gentlemen. I've been really confused lately because of all the recent errors I've been getting. Okay, so basically I run a home-hosted server, and for some reason, at random times I see about.. Roughly four hundred lines of "Unknown command please type /help" thing.. Then... Obviously, the server crashes. I don't know the cause of this. This is the console:
(A lot of "Unknown command. Type "help" for help.")
Then...

[20:04:40] [Netty IO #1/WARN]: Unexpected exception in the selector loop.
java.lang.OutOfMemoryError: GC overhead limit exceeded
[20:06:37] [Server thread/ERROR]: Exception stopping the server
java.lang.OutOfMemoryError: GC overhead limit exceeded
[20:21:04] [Thread-1/WARN]: Exception in thread "Netty IO #2" Exception in thread "Thread-1"
[20:13:56] [Netty IO #4/WARN]: Unexpected exception from an event executor:
java.lang.OutOfMemoryError: GC overhead limit exceeded
[20:18:45] [Thread-4/INFO]: Stopping server

Don't know what could be causing this, but the first line before all the Spam comes in is..

Usage: /say <message ...>

P.S Sorry for my bad English, I'm Spanish.

 

The spam is causing that.

What plugins do u got?