Your PayPal information is not actually stored on Plarium servers. If you don't want your PayPal account to be used for further purchases, you can simply reset Plarium Play's cache. Please follow these steps:
1. Open the Settings tab in the Plarium Play launcher
2. Click on "Update"
3. Choose Reset cache
Your PayPal information will be gone after the cache reset.
Please note that you will also be logged out, and will have to log back in.