Doesn't runesmith or dfhack have a utility to mod starting dwarf amount? Not sure off the top of my head. It won't prevent the first two waves, but setting pop cap to 1 and having those waves kill themselves (or doing it with said utilities and laying them to rest, again with said utilities(unless you remove them from existence entirely, but I don't know if you can do that.) and slabbing them will probably be effective. As long as you aren't in an evil biome, of course. Or you could just roleplay it that your dorf is the last one not infected/cursed by something or another, which would work well. Just gotta make sure there's no ghosts, because having your only dorf murdered by a ghost isn't fun enough.