Warning: Once a database has been deleted, there is no way to reverse the action unless you have a backup that you can restore the database from. Please be aware that it is your responsibility to keep an up to date back up of your databases as your 123-reg hosting package does not come with a backup service.
You can delete a database from your 123-reg hosting package through your 123-reg control panel:
Scroll down to the Web hosting section.
Select the relevant domain name from the drop down box and click on the Manage button.
Click on the Database link in the Web Tools section of your hosting control panel.
In the Manage Databases section tick the box for the database you wish to delete and press the Modify button.