Zoiper3 for Windows keeps the contacts in a file named Contacts.xml
You can remove this file by following these steps:
1. make sure zoiper is not running;
2. open windows file explorer;
3. type %APPDATA% in the address bar;
4. press Enter
5. Open the folder named Zoiper;
6. Rename the file named Contacts.xml
Next time you start Zoiper there will be no contacts.