As a global affector, you could try playing with the THROUGHPUTTimethreshold
server option, specifying a high THROUGHPUTTimethreshold value, which your
clients could not possibly achieve, thus causing any client session lasting
more than THROUGHPUTTimethreshold minutes to be cancelled automatically.
I haven't tried this myself, but it looks like it would work.

   Richard Sims, BU