There are some forums about this on Beta
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There is a button at the bottom of the page that says follow. If the box is checked, then you are following it. Uncheck the box to unfollow it.

There is a button at the bottom of the page that says follow. If the box is checked, then you are following it. Uncheck the box to unfollow it.
but if the OP blocked you, then the box disappears
How can I unfollow a topic that I am blocked from?