If you have full admin rights in-game, type these commands at the console:
amx_restrict on weapon
For instance, to disable the shield, I might use "amx_restrict on shield" followed by "amx_restrict save". To disable the awp, I would replace the word "shield" with "awp". The abbreviations are the standard weapon abbreviations used by Half-life. (To un-restrict a weapon, use "amx_restrict off weapon".)
You can also place these commands into the server.cfg or the amxx.cfg to ensure that its permanent.