Best is to start from scratch:
1. Uninstall blastchat component.
2. Delete tables blastchatc and blastchatc_users from your database
3. go to
www.blastchat.com and enter "Your registration"
4. Select your website
5. Delete your website registration (all rooms and users information will be deleted)
6. Install BlastChat component and register it from joomla/mambo backend (i.e. completely fresh installation)