stretch copper wire from stripped electic power cord across a board and electrify it, to the voltage of your choice.

Placed just inside the cat flap whilst your cats are confined for an evening, it should deter any future visits.

It worked with a dirty tom cat that was coming in and fouling/spraying in our house. One experience was enough for him.

Our electicity supply is 220 volts, but we did not give him the full amount.

More details of the construction of the deterent can be supplied if interested.

Anne