This will check whenever someone gets hurt, and if whoever attacks them is in noclip, it will turn off the attackers noclip.
I've attached the file, just place in <GMod dir>/lua/autorun/server/
function NoClipDamage( vic, att )
if att:GetMoveType() == MOVETYPE_NOCLIP then
att:SetMoveType( MOVETYPE_WALK )
end
end
hook.Add( "PlayerHurt", "NoClipDamage", NoClipDamage )