that's because it's a server side option only.
you as a mapper have no say in the matter. unless you strip the player of all weapons and allow them to choose from a selection of
armoury_entity designated weapons at the start of the round, ala fy_iceworld (and the millions of others
)
however this causes problems in that your map will be sh!t. the awp is a weapon in the game, you can design your map to be biased towards
not using sniper rifles which means the clever player will use assault rifles and such as opposed to awp etc.
short answer: can't be done