I want to completely cancel the return segments. I don't mind "losing" the ticket. I just don't want to be charged for not taking the return flights. And although it "doesn't make sense," the rule definitely specifies that they have to charge me a change fee for abandoning the return portion of the ticket; I'm just wondering how strictly that rule is enforced by the reservation agents.

I want to cancel the return portion, rather than just no-show, because I may be doing other UA flying before the return portion, and it may trigger automated checks of something funny going on.