I also recommend going the route of "this conversation ends NOW." It depends on your mom's personality type whether it's effective right away, or uncomfortable but at least it gets you away. You also do have to be very vigilant about it.
A couple other things you could try. You could try the Serious Conversation again. Sit her down alone over a cup of tea and tell her that her fixation on you having kids has negatively impacted your relationship with her, and as a result you have chosen to spend significantly less time with her. Don't let it become about whether or not you're having kids. The issue is that she's constantly bringing up a topic you told her was closed, and that needs to stop. It could be her going on about your job or your hair color or your house or anything--anything where you've told her the matter is closed and should not be discussed again, and she's disregarded your wishes.
You could try remarking casually, "Mom, perhaps you don't realize this, but you've brought up the issue of grandchildren in every conversation we've had this month." Maybe she really doesn't understand how often she's mentioning it and will stop if you point this out.
You could also try taking a sympathetic approach. Since you are the one in control of your reproductive habits, not her, you can afford to be magnanimous and perhaps chat with her about baby names she likes, or what trips she would take the kids on, or things like that--but all with the underlying tone of, "Well, that's a nice fantasy that's never going to happen." Maybe giving her some room to breathe on this subject will take the edge off her longing. You could also encourage her to spend time with other small children in the family, like your cousins' kids, or to volunteer somewhere in the community with children--T-ball, children's library, preschool, crisis nursery, etc..
Obviously these approaches depend on your and your mom's personality--for example, the sympathetic approach would be bad if you're sure it will only make her hope harder, or obnoxiously "talk up" kids to you in an unrealistic way. But, if you think she will take it rationally, it might help you both understand why she wants grandkids so much--does she want to see the family extend itself (in which case perhaps she could transfer her affection to cousins' kids or, more indirectly, get into genealogical research), does she want to cuddle babies (volunteer at crisis nursery/infant hospital), etc..