Unless you put a date on the referral form stating otherwise, it doesn't expire. What if you were the one who extended the referral to another agent? Should it expire--or should it remain in place until that agent finds what the person wants or until the listing sells? I'm a big believer in do unto others as you would have them do unto you.
As Elizabeth said, a referral is a client that you would have never had if you didn't get the referral. If you don't want to work with the clients anymore, then drop them and you don't have to pay the referral. If you want to work with the client (which you would have never had if not for the referral), then pay the referral fee.
Work hard for the client. If they are happy with your services, they will hopefully refer some friends and family to you, and you don't have to pay a referral fee for those!
If you don't want to work with the client, and as long as you are not in escrow, you can resign your position and move on. But as long as you are working with the client (whom you may not have picked up were it not for the referral) then you should honor your agreement.
I completely agree with Elizabeth. This business is a give and take. There are going to be times when you curse the fees but then you are rewarded for your excellence in service when your client refers you to their friends and family. Stiff upper lip and know that you are doing a great job or you wouldn't have been refered in the first place.