WM search strategy

I want the system to place a material in a bin,in such a way that the bin should contain only one material code & different stock type like Q,S & R only.

It should not allow multiple material into the bin.I have used manual placement startegy with user exit.

The user exit works perfectly, when i dont give the bin while creating the TO.The user exit is bypassed when the bin is keyed in manually & it allows all stock type & all materials in the bin proposed manually.

How tomake the system to throw error message when the user exit is byapssed