Long version:
Worst part of this situation is your eagerness closes your own options. If a dog is not interested and you keep playing (and I mean playing, not "playing") with him, it is fairly possible, at some point, he'll get the idea.
But... if you insist to the point he needs to go away growl/bite... then it becomes something negative to him and will likely never be interested.
Or to explain more grafically:
If you are playing with a dog, or scratching or whatever he is enjoying, then you reach "back there" a bit and he stops and looks at you there are two options:
1 - You ignore it, do not acknowledge the look and keep doing the play as before. This will come to him as "ok nothing wrong intended" and with time *might* get comfortable with it or get itvas part of the play (or maybe not. Ever.)
2 - You stop playing, look at him, grab back "back there" cause you are the master and he *should* learn your touch is not going to hurt him ever. (Bonus points if he needs to snap at you so you stop it)
Tip: Number 2 is not a good option and removing a negative conditioning is really hard.