Cannot remove related forum thread from entry

I was setting up a new section on my website and inadvertently inserted the wrong forum thread ID number to relate it to an entry. Now, I would like to just delete the relationship between the entry and the forum thread entirely, but it won’t allow me to do it. Whenever I delete the forum thread ID number from the entry, there’s a pink exclamation point on the Forum tab on the Edit Entry page, and the link to the thread remains on the entry.

ExpressionEngine version 2.9.0 Discussion Forum module version 3.1.16