1. Stop the "Archive Manager Message Retention Policy Service" service.
2. Open the RPS.wlog from your logs directory and find the retention run that archive manager is currently working on.
3. In the following log entry we can see that retention is currently working on run 866,
2015-08-02 20:19:35,679 [7] INFO BL.RRun - Retention Run 866: applying policy
4. In SQL Management Studio run the following command using the value from above.
USE ArchiveManager
EXEC RetentionRunAbort @retentionrunid = 866
5. Start the "Archive Manager Message Retention Policy Service" service.