You can blocklist content on your Skynet portal. The blocklist will prevent skylinks from being downloaded. Since Skynet blocklist by merkleroot, this will also prevent uploads of the same underlying data (and therefore pinning the files) that would resolve to the same skylink and merkleroot.
The command for blocking a file using siac is
siac skynet blocklist [skylink] [flags]
For example, you want to blocklistsia://ABCdefGHIjklMNOpqrSTUvwxYZ12345678901234567890 or its web-friendly form https://siasky.net/ABCdefGHIjklMNOpqrSTUvxwYZ12345678901234567890 you would run the following:
You can also use the Sia API to add or remove multiple links at a time. This is especially useful for portals that want to share blocklists with each other. For more examples, refer to the Sia API Docs.