Right now it's her status that's trivializing floors more than her other options. So adding more characters like Gilder or other characters who can shut down status should help. Hand out Ribbons or Status Symbol Law sealstones to characters in certain fights. If you really wanted to get creative you get incorporate more characters who shut down item use like Bowyer or say a Weavile (from Pokemon) that knows Embargo. It's the same problem we've always had with characters who have accurate status or ID.
Well, I wouldn't say it's just her status. I mean, the super-accurate-lethal-MT-status-that-comes-in-different-flavours is definitely part of it, but I know a lot of people see Rikku as being able to charge her Overdrives by leaving the weakest opponent and alive and letting her hoard turns. And Overdrives like Super Mighty Guard off her crazy first turn speed can certainly trivialize a fight as well. Rikku murders PCs, but can easily trivialize boss battles, too.
I don't know if I like the idea of making certain fights completely status immune, or unable to use items, since I feel it'll hurt a lot of teams, all for the sake of shutting down status users. I mean, there are others with good status, like you mentioned, but Rikku has batshit crazy first turn speed. Songstress borders on average speed (and is really fragile), while someone like Nash's status is arguably not even MT. When it comes to status use in the dungeon, the primary objective is to prevent the opponent from getting a single turn. Most people in the dungeon don't live beyond a single turn, unless you're a durable boss, so if you can't prevent that single turn, status isn't really that worthwhile. Rikku excels at it thanks to her speed. And when she can't nuke with her status, she's got an actual backup strategy with Overdrives.
I mean, she's definitely worthy of being a 4.0, but I can understand Neph's gripes. =P