There is no way I know of to prevent Source server from showing kick and banned messages.
However, ULX does have it's own punishment messages.
Short answer -
Release: server.ini
SVN: Not working
Long answer-
Release version : see addons/ulx/cfg/server.ini
After you edit any of it's logecho variables (or anything within it), don't save it back to addons. Save it to gmod/cfg/server.ini
SVN revision: Current SVN doesn't have control of any of the convars formerly used in server.ini
They will be added back as time allows.