The easiest way to prevent erections after a circumcision is not to be circumcised in the first place!
You do not need to be circumcised just because you have a tight foreskin and, according to you, cant pull the foreskin back. Who ever told you such a load of rubbish!!!
The foreskin offers you protection against physical damage and infection, so why would you want to get trid of it? Would you cut off your eyelids just because you get grit in your eye? I doubt it, because you realise the natural protection that your eyelids provide. Same with your foreskin.
It is totally possible to live like millions of other men who have tight foreskins (myself included). They can masturbate or have sex. They can father children or engage in same sex relationships. They can live a life free of the devastating problems caused by circumcision, which is, by the way, a rather serious and dangerous medical procedure that results in numerous damaged penises and even death. And, as I said, there are millions of guys just like you making your "problem" quite normal after all.
All you suffer from is misinformation.
Rethink your problem before it is too late. Your foreskin is a very valuable asset and you really need to guard it with your life.