1. Switching back to default theme
Go to the admin area and then system > configuration > design
Change "Current Package Name" to 'default' and make sure all inputs under the "Themes" section are empty. This will return you to the default theme.
2. Completely remove everything added by this theme
I take it from your question that you are not using any form of source control. Might be an idea to look into this for the future :)
Magento 1.7 has introduced backup and rollback features so you may also want to check that out.
If this is the case, backup everything before you start removing files and directories as described below
Anyway, to completely remove all files and directories added when you installed the module, unfortunately it will simply be a case of manually finding the files and directories deleting.
The easiest way to do this will be to find the original source code you received when you purchased the theme and removing the corresponding files from your site. When removing directories be careful to only remove those specific to the theme - It will have been sent to you structured so that you can easily drag straight in to your web root - so will contain some other directories required by Magento, simply to give it the correct hierarchy.
At the very least there will be files and folders specific to the theme located in the following directories:
- app/design/frontend
- skin/frontend
- js
and also most likely some more in the following:
- app/etc/modules
- app/design/adminhtml
- app/locale