You can buy some animal pir's they are less sensitive than the normal ones, somethin to do with the size of the object that they sense, had some fitted to my system when it was installed.
the other way round it is to put some tape on the bottom of the pir's you already have this will then only sense any object over a certain height, not much good when the little buggers are crawling up the curtains which has happened on a few occasions
