Organizing and accessing contact information in Outlook.com
Hello, MarekSzczerba
Welcome to Microsoft Community.
If you need to delete a large number of contacts, you can click on one of them in Outlook, press Ctrl + A at the same time on your keyboard to select all of the contacts, and then delete them all at once.
To delete all the items from the Deleted Items folder, you can empty the folder in Outlook.
If you can't select all the contacts or empty the Deleted Items folder, please let me know, and I'll keep working on this issue.
Best regards
Yuhao Li
Microsoft Community Technical Support