The purpose of RespectMyPrivacy is to keep your contact information out of the public WHOIS database while still permitting you to be contacted if necessary. Expired domains persist in that database, complete with publicly visible contact information, for a really long time before they are finally deleted.
ICANN does not allow changing the contacts on an expired domain, so it is not possible to remove privacy service after expiration and before deletion. Once your domain is deleted, RespectMyPrivacy will be automatically canceled.
To remove RespectMyPrivacy from a domain that is not expired, see this FAQ entry.