By chance this same subject has been on our company intrantet. Here is one of the suggestions for scaring cats away
Ca####ch cat deterrent
Very effective and the only cat deterrent product tested and recommended by the RSPB. The sensor detects movement up to 12 metres away and then emits a high pitched sound (silent to humans) in a wide arc, which is pitched specifically at a cat's hearing frequency. Unit height 14.5cm. Uses one 9 volt battery (not supplied) or a 12 volt mains adaptor (supplied separately). For long term economy the optional mains adaptor means you will never have to replace the battery in your Ca####ch cat deterrent again. Fitted with 9 metres of bell wire to enable you to position the unit where you want it.