I've had the same problem on my server and ended up restricting it to guests, I never thought of using a script to prevent it from being used on players. I've just started scripting so I'm not sure if this will work:
function UseTool( ply, trace, toolmode )
if toolmode == "material" or toolmode == "colour" then
if trace.Entity:IsPlayer() then
return false
else
return true
end
end
end
hook.Add( "CanTool", "UseTool", UseTool )
Edit by Megiddo: Oops, accidentally hit edit instead of quote. Sorry about that!