Stopping cat from keep coming round ?!

[StandingInLine]

We had a stray not too long ago which we kept for 3 weeks. In this time I had him checked for a micro chip which he didn't have ,left details with vet in case someone claimed in. 3 weeks later I get a message from a someone in another garden (our gardens back into each other ) asking whether we had her cat as someone had seen him in our window and I have him back.
However ,now I'm having to take him back every day as he's coming over ,using my cat flap and playing with my other cats. Today the owner has asked me not to feed him which I haven't purposely but can't not leave any food out as I have 2 other cats ,and can't lock the cat flap as it's not fair to my cats for them to be locked in or out.
What can I do ?!

Fit a microchip cat flap - it only allows in cats who have 'authorized' microchips. Sureflap is the brand most posters use. At just under £70 online, it's not cheap but it's a good investment - if you have an ordinary cat flap you'll soon find yourself on the local Tom Radar generally and not just for the cat over the back.

\link{http://www.amazon.co.uk/SureFlap-Ltd-Microchip-Door--White/dp/B003EGIM3O/ref=sr_1_1?ie=UTF8&qid=1383864130&sr=8-1&keywords=cat+flap+sureflap\Here's} a product sample. Note - there may be cheaper ones out there.

Your other option is to be a little more heavy handed, put flap on in only, then in front of cat perform a loud drumming routine with saucepan and metal implement. Basically scare it a bit. Only let it out once it looks suitably frightened. Bit cruel but if it has a nice home elsewhere then he needs to go.