Is it permissible for a wife to prevent her husband from fondling her anally if she fears that it might escalate to forbidden anal intercourse? And does she deserve the curse of the angels if she does so?

Touching the outside of the wife's anus by the husband is permissible as long as it is not thought to lead to anal penetration. If such a thought arises, then approaching it becomes forbidden, as it is a means to something forbidden. The wife must then abstain, and there is no sin upon her, nor will she incur the threat mentioned in the Hadith. Rather, it is obligatory for her to do so as a preventive measure.