It is possible to do this but you know you are going to have to spay your cat eventually. Along with preventing kittens spaying also prevents breast cancer and all the other cancers unspayed cats get.

So I looked this up in a book and will give you the instructions they give to stop oestrus (heat cycle)

"A queen can be brought off heat by stimulation of her vagina. An assistant should hold her by her scruff and deglect her tail while you insert a q-tip about 1cm (that 3/8 of an inch) into her vagina and gently rotate it. The queen will struggle and may call out; this is quite normal. She may also show typical post coital rolling behaviour. She should cease calling in a few days". They also recommend a teaser tom if available.

This isn't going to work forever and realistically you need to get your cat spayed.

This from "The Book of the Cat" copyright 1980 byNew Leaf Books Ltd. of Great Britain

ISBN 0 330 261533