Hello Kalihaz,
Thank you for posting in Microsoft Community. I’m happy to assist you with this request.
Unfortunately Outlook web limit contact deletion to 10 contacts per time. However, you can log your account from any desktop app and delete it all in one shot as the following:
1- Navigate to people
2- press CTRL+A for Windows or CMD+A for Mac to select all
3- then, delete.
4- Sync. (this is the most important step)
Please do not hesitate to contact me with any questions or concerns. I look forward to hearing from you.
Please understand that our initial response may not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.
Best regards,
Community Moderator