The best approach in this situation is honesty. Let the person know straight up that you aren’t ready to go any farther. Hopefully, he’s concerned about your feelings, too, and respects what you are ready for.
A relationship is a mutual partnership, and if one of you isn’t ready, the couple as a whole isn’t ready. Everyone progresses at different paces in a relationship, and each relationship is different based on the two individuals involved. Something special like physical contact is private and shouldn’t be embarrassing or even hurtful if one of you isn’t ready.
At times like this, being polite should be the least of your worries. Being diplomatic is all you should be concerned about. Start by gently pushing him/her away and saying that things are going father than you want. That should work. If it doesn’t, your partner isn’t being polite and you shouldn’t be too bothered about doing so either.
Push harder, and be firmer in what you say, stand up and move away from him/her if necessary. If /she is really unyielding, you may have to get angry to be heard. Don’t be shy; this is your body and you are the only person who gets to decide what to do with it. Chances are very good that your partner will stop when you first put up the stop signals, so it is unlikely that things will have to get ugly. But if you do need to assert yourself to make the other person stop, do not be afraid to do so, living with the fact that somebody bullied you into being more intimate than you wanted to be is never a nice thing.