I dont know whether such option already exists, atleast I didnt explore it yet.
This basically would be an option especially for weapon modified servers, like knife only, saw&law etc
Why i'm suggesting: it's just very annoying sometimes when you misclick a couple times in a row or often in a game whilst there only is 1 weapon you want to play with (or the server only has 1 weapon allowed)
However, There should also be some button or combination that stops the automatic picking of a weapon, because if someones does choose to pick another weapon they shouldn't have to rejoin or whatsoever.