This week, my neutered middle age male cat called Garfy brings home a 'guest', who is a much smaller cat. I guess it's very young, but not sure of the gender or origin, so let's just call it Taby. I saw Taby a couple of times hanging out in our garden with Garfy. They seem to be good playmates.
The problem is that Taby often sneaks into the house through the flap door after Garfy. Then not only s/he eats Garfy's food, but also makes a lot of noise during his futile exit attempts, because opening the flap door is a bit tricky as it opens only towards inside. So I have to wake up early on and let Taby go by opening the entrance door, which is obviously not my favorite morning ritual and deprived me of sleep several times.
I'm not sure what Garfy feels about Taby's sneaking in, but I know that Garfy is so passive that he is frequently being bullied even by much smaller cats. Garfy's only line of defence against intruders is to scream. So I imagine he just tolerates Taby's entrance, or even encourages it as an act of philanthropy!
To make it clear to Taby that it is not welcomed inside, I rush to the entrance door and make some threatening noise and gestures before letting him go. But that was not effective and I suspect s/he will be back tomorrow.
So I'm wondering how can I deter/dissuade this unwelcome guest with the least harm/hassle?