RESET SLAVE
        RESET SLAVE makes the slave
        forget its replication position in the master's binary log. This
        statement is meant to be used for a clean start: It deletes the
        master.info and
        relay-log.info files, all the relay log
        files, and starts a new relay log file. To use
        RESET SLAVE, the slave
        replication threads must be stopped (use
        STOP SLAVE if necessary).
      
          All relay log files are deleted, even if they have not been
          completely executed by the slave SQL thread. (This is a
          condition likely to exist on a replication slave if you have
          issued a STOP SLAVE statement
          or if the slave is highly loaded.)
        
        In MySQL 5.4 — unlike the case in MySQL 5.1 and earlier
        — RESET SLAVE no longer changes
        replication connection parameters such as master host, master
        port, master user, and master password. (This means that
        START SLAVE can be issued without
        requiring a CHANGE MASTER TO
        statement following RESET SLAVE.) However,
        connection parameters (which are now retained in memory even
        after RESET SLAVE is issued) are reset if the
        slave is shut down.
      
        If the slave SQL thread was in the middle of replicating
        temporary tables when it was stopped, and RESET
        SLAVE is issued, these replicated temporary tables are
        deleted on the slave.
      

User Comments
If you want to make a slave *completely* forget that it ever was a slave, then:
CHANGE MASTER TO MASTER_HOST=''
seems to do the trick.
If you run CHANGE MASTER TO MASTER_HOST='', you will not get any output from SHOW SLAVE STATUS. However, if you look in the filesystem you'll still see the master.info files, etc. If you then run RESET SLAVE, these files will be deleted.
Add your own comment.