If you don't believe me, here's a screen shot.
Anyway, I did a Google search and found it has come up before, so I'm not the only one. But not very often, which seems to indicate there is no user interface for it. Very odd.
BTW, "just blocking them" is not an answer, because a friend who I don't want to unfollow frequentlly RT's the person who has me blocked from RT'ing. If it gets annoying enough I guess I will have to unfollow the friend who I don't want to unfollow.
In case it isn't obvious, Twitter is a morass of inconsistent rules about when it should and shouldn't show you stuff, and makes you block people when you have no interest in blocking them just to get another feature that comes along with blocking.