If the server allows them to use noclip, then they can use noclip.
when noclipping, they do not physically touch areas, so even triggers may not fire or act upon a person correctly.
Additionally, placing a trigger outside the map would result in a leak, unless you do the evil box routine around the map, but that'll be hideous for gameplay and compiling.
Looking it up, however, you may be able to set these variables on your server using a point_Servercommand: sv_noclipspeed - The maximum speed the player can move during noclip. Default is 5. sv_noclipaccelerate - The speed at which the player accelerates during noclip. Default is 5. Set them to zero (0), meaning the player will be unable to move when noclipping. Trigger this entity with a logic_Auto "onMapSpawn", or to prevent them changing it later on, via a logic_timer with a low time (but greater than one second, else the spam... the spam!!!)