I have to agree with Tomato. Firing pellets at the puss seems hellish cruel to me.
We used to have this problem until one day Mr.C was pressure-washing the car and fired a quick blast in Puss's direction. Puss was quite a distance away and I think he was more startled than hurt.
I went mad at Mr.C at the time as I hate anything like this, but it did stop the puss using our garden.
Please don't blast the puss from close up though, that would be horrible. You want to frighten him off, not hurt him.