When I worked for the University of Hartford, we had the same problem (in
mass proportions).  Our solution was to tape a notice across the ad
stating the University's policy on theft and mutilation of library
materials.  Underneath the notice we placed a Knogo strip (tattle tape).
The idea was that if the "interested parties" were ignorant enough to
disregard the notice, we might catch them at the door.   After the first
notice, however, the back covers stayed on.  I left in 1992 and I don't
know if UofH still does it.

Paula Lynch  (SILS, UNC-CH)
lyncp.ils@mhs.unc.edu   - or - saspzl@mvs.sas.com