If a wife no longer wants to have relations with her husband, is it allowed for her to stop going to the mikva, in order to prevent relations?
A woman who prevents her husband from having relations with herself, is a מורדת - and this is grounds for divorce.
Details can be found in the Rambam - הלכות אישות - Ch 14 (Halachot 10 - 15) and in Shulchan-Aruch Even-HaEzer Ch. 154:3-6 (למי שכופין להוציא בגט)
Rabbi Avigdor Miller writes in his shalom bayis manual, Career of Happiness, that a wife should never cease going to the mikvah because of marital difficulties. However, I'm not sure if it's permitted halachically.