First, best way to prevent chat spam, don't use chat to run commands. Chat isn't meant to be stopped.
Use console commands. They autocomplete for you and can usually be done faster than chat commands.
Second, re: console commands -
If you've not already, take a look at your <gmod_server>/data/ulx/config.txt.
It has many options, including what is echo'ed to clients.
If you don't have a config.txt there, COPY (not edit) the config.txt file from <gmod_server>/addons/ulx/data/ulx/ to your <gmod_server>/data/ulx folder then look for/edit the section that looks like below.
ulx logEcho 1 ; Echo mode
; Echo modes:
; 0 - OFF No output to any players when an admin command is used
; 1 - ANONYMOUS Output to players without access to see who used the command (admins by default) similar to "(Someone) slapped Bob with 0 damage"
; 2 - FULL Output to players similar to "Foo slapped Bob with 0 damage"
There are also several other logging options you may use to help. Look over them, consider them carefully...many things logged, if turned off, may be a detriment later.
Third - Use XGUI with second part modification above.