1. Go to Setup>Sites

2. Click into your site and click edit

3. Change Active Site Home Page from Checkout to InMaintenance, delete URL Rewriter Class so the field is blank, then click save.

4. On the site page, scroll down to Error Pages and click Page Assignments.

5. Make sure no pages are set to Checkout. Use InMatinenance if in doubt as a default setting.

6. Go back to your Site Details page and click Public Access Settings

7. Click View Users

8. Click into Payments Profile

9. Click Edit Assignments

10. Remove any assignments in Enabled Permission Sets. Alternatively, just delete any permission set assignments. 

11. Go back to Setup > Installed Packages, click Uninstall on Payments 2 Us

12. Scroll down to the end of the page, and click Yes I want to uninstall this package, etc, then the Uninstall button. 

13. Unless you have already deleted all custom fields, campaigns etc, a list of components should appear as an error. Delete all items listed (you should be able to click directly into them). Once everything has been deleted, repeat steps 11 and 12.