Hi Lambros,
Thank you for posting your concern and feedback regarding Outlook.com Calendar.
Please be informed that once you've managed to connect your social networking accounts to your Outlook.com account, all information such birthday events, contacts will be added to your Outlook.com account and the only way to remove this to your Outlook.com
account is to remove all associated services. You may follow the steps provided by Christian_N for proper steps about this.
You may also visit this link if you have any other suggestions or comments about the product.
These feedbacks are collated and reviewed and can be a candidate for future releases.
If there's anything else that I can help you with, please do not hesitate to reply here and we will further assist you. We'd love to hear from you again.