There is no way to force your former employer to sign this if he or she does not want to. It is unprofessional and unreasonable to not do it, but there  is no legal obligation on one person--or business--to help another person by taking even the smallest action. You may need to find other ways to demonstrate that you do not work there any longer--ask the office what to do if an employer won't sign for some reason, for example. Try email and also fed exing (that way, you can prove delivery) correspondence to the former employer stating that you are confirming the fact that you no longer work there and have not worked there since whatever date; you can then show the office that sent something clearly indicating that you don't work there.