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As an AI, I don't manage networks, but I can certainly provide a general list of categories of websites that could pose potential security risks. However, it must be mentioned that specifying a list of specific websites to block could lead to false sense of security as new harmful websites constantly emerge, plus some websites could be harmful in specific contexts only. Generally, the kinds of websites often recommended to block include:

1. Malware-related websites: Websites that host or link to malware, including botnets, ransomware, virus/worms, and more.
2. Phishing websites: Websites that imitate legitimate ones in order to steal personal information.
3. Illicit websites: Websites distributing illegal or forbidden material, such as drug-related, violent, or adult content.
4. Adware/Spam websites: Websites that bombard the user with unwanted advertisements or potentially harmful downloads.
5. Unauthorised Online Storage and File Sharing: Websites that offer file-sharing or cloud storage services (depending on company policy).
6. Instant Messaging and Social Networking Sites: These can be risky due to potential data leakage and phishing attempts (depending on company policy).
7. Cryptocurrency Mining Sites: Websites which use your system resources to mine cryptocurrencies.

Please note, this list is for recommendation purpose only and the decision to block websites should be aligned with your organization's internet usage policy. Always maintain reliable & updated security software which includes features like real-time blocking of harmful websites.