Change your password so the surprisee can't see it?

If they notice, and you need to come up with a story about why, just say something like I'm increasing my password security with all the (mostly non-UA) hacking, etc. going on recently.

You can remove them from the app, IIRC, but not from the main site.

You could also try the 'combine' function if you have another reservation that might 'hide' it within another itinerary, but I'm not sure exactly how that shows up on the main page, and if the surprisee goes into see the details of your itineraries, could find out that way.

Or, you can just book without logging in, and without adding your MP number. Then it won't show up in your account. Or if you have someone else in your family with an MP account, book it through that, again, without your MP number on it. Then add it whe you get to the airport, or closer to the departure time. Note not putting your MP number on will prevent you from getting any status/other benefits you might have, like advance CPU, UA.com club credit, assigning E+ seats, etc.