How do you remove the supply of stolen tools?

You can have trouble to with the gang : Their cells are not searched with theim of the other inmate.

To search in the cell of a gang, you need to: The gang member have to be out of this cell

AND

you have to select the gang member one by one, and select the action "search on the cell".

BUT

It will not search on the prisonners himself, so you have to select too the action "search on the prisonner"

Assuming you are finishing the grant related to this task, a shakedown should be able to complete the grant, it has for me all times I've attempted it.

To prevent future contraband from coming into your prison you can build visitor booths instead of visitor tables in the visitation room, which will remove all contraband income from the visitation room but won't reduce the family need as much. You'll also need to station guard dogs near an infirmary or the psychologist's office to stop drugs from being smuggled.

I also suggest having metal detectors at the beginning of the cell blocks and in front of the entrance of the canteen, I highly suggest you get a reception as well, because this will remove all contraband from new prisoners, but in return you'll have to be more active with laundry.

If you're short on time and you've got a lot of ground to cover, you can just mark all of the rooms that have contraband (workshops, laundry, cleaning cupboards...) as staff only and lock their doors, then do a shakedown to get rid of the hidden stuff. Works like a charm.