A server side mod that would make your game sniper only is essentially a .pk3 file that you place on the server, in the mohaa main folder.
within that .pk3 file contains various .tik files. Each of those .tik files defines the characterisics of each weapon. In the case of a snipers only mod, the .tik files would tell all other weapons to have an ammo count of zero. some other sniper only mods allow sniper rifles to carry more ammo in the clip.