To uninstall Payage, use the uninstall option in Extension Manager.
If you uninstall a gateway addon and Payage accounts exist that are of that gateway type, the accounts remain in the Payage accounts list but cannot be used until you re-install the gateway addon. You can delete the account if you no longer need it and there are no payments associated with it.
When you uninstall Payage, the files are deleted from the file system but the database tables are not deleted. To remove them you can use a database tool such as phpMyAdmin.