Kavitha,
In current versions (9.12 and older), you cannot purge messages, but you can achieve the same effect by deleting the Named Object (= durable subscriber) associated with the subscriber. If you then disable/enable the associated client, trigger or connection, it will automatically recreate the Named Object, which will be empty.
In the upcoming 10.0 release (April 2017), you will be able to purge all messages or individual messages.
#Integration-Server-and-ESB#Universal-Messaging-Broker#webMethods