If they are under 18, that baby is going up for adoption. If it was a son and the girl he got pregnant refused to, then I would take it court and make it very clear that, it is our intent that this baby is put up for adoption and if she refuses and decides to keep it, we waive all responsibility for that child.

If it was a daughter, that baby would be carried to term (as I believe abortion is wrong) and then would be immediately put up for adoption irrespective of her wishes.

If the child was 18-22, I'd encourage them to seriously consider adoption to the point where if they refused I would at least temporarily write them out of my will, but once they are of adult age you can't make them do something directly, all you can use are financial and social consequences that you can bring down to try to indirectly influence them.