Give them a lie detector test. If they refuse to take it, fire them.
I had to take one to get hired at a high end luxury apparel store. The need for one at the WH exceeds the one I had to take.
All high level White House staff, and all staff who have access to TS-SCI, should be polygraphed yearly.
If they fail they should lose their position and be investigated. If they are convicted of leaking classified information they should be shot. This isn’t complicated.