Family Lore Clinic: What do I do if my ex refuses to sign the decree absolute?
I will assume here that the questioner means "sign the application for" the decree absolute, and that his/her ex is the person in whose favour the decree nisi was pronounced.
The answer is essentially quite simple: you cannot force your ex to apply for the decree absolute. However, you can yourself apply for the decree absolute, once three months have elapsed since the earliest date upon which your ex could have applied, which would have been after the expiration of six weeks from the date of the decree nisi (i.e. you can apply after 6 weeks + 3 months from the date of the decree nisi).
It should be noted, however, that such an application cannot simply be dealt with through the post, as would an application by your ex. The court will fix a hearing and may not grant the decree absolute, for example if it considers that any financial/property settlement must first be finalised.
(As usual, if you require more details or specific advice, you should consult a specialist family lawyer.)