it's rather simple.
All you have to use is the forward CS_OnBuyAttempt(id,item) and check if the item is a CSW_SMOKEGRENADE (constant)
That way you can block out anyone from buying smoke grenades.
What is important is that you have to write the code this way:
Return
PLUGIN_CONTINUE to let the buy attempt continue
PLUGIN_HANDLED to block the buy attempt
That means if the player is trying to buy a smokegrenade (which you check with an if-condition) you just return plugin_handled. IF the player buys something else you return plugin_continue.
Have fun
Vancold
Remember that this works only in AMXX 1.9 or above.