I just got my 7 month old cats spayed last Saturday. She's completely fine now - doing all things cat.
But last night I woke up to the neighbourhood 9 month old tom cat humping her on my bed! He came in the cat door.
She didn't seem too annoyed but I shooed him away & locked the cat door. Today I saw him 5 times LEAP onto her, grab her by the scruff of the neck & growl.
She still has that female smell, spayed or not. The tom cat needs neutering by the sounds of it, or you'll have no peace. I've got the same problem with a local tomcat harassing my two females.
Do you know whose cat he is? If you can collar hold of him, give the local branch of either cats protection or rspca a call, and ask them for a neutering voucher. Then take him to the local vet and get him done.