So it seems strange that you can't have office hours right after this lecture, but you can, in fact, spend 15 minutes after the lecture time helping students.  It appears you could have 15 minutes of office hours right after this lecture.  Perhaps the travel time is excessive, or you have an aversion to setting up office hours of such short periods of time?

An option not already presented in other answers is to end your lecture 5-10 minutes early and encourage students to spend the remaining time working with each other, or asking you questions individually.

This would alleviate the issues you have with limited office time. I don't think it's a great choice, though, as lecture time is also valuable.

I think to root problem is convenience.  You find you can support your students better by providing some time at the end of class, which they already attend, to support them rather than expecting them to schedule time out of their day to visit you during normal office hours. You don't seem to suggest you don't have enough office hours, so it appears you are doing this purely as a convenience for your students.

You don't have a right to the classroom outside your lecture hours, save, perhaps, for several minutes before and after to set up the classroom if necessary.  Your use of the classroom as an informal office is a convenience.

If your university or college doesn't have rules regarding classroom use between lectures, or preparation/cleanup time, then I'd suggest it's fair for you and your students to vacate the room by the halfway point between your two lectures.  This doesn't mean ending your conversations ten minutes after your lecture, but being out of the room.

I suspect that she's only becoming more and more insistent as you are consistently going well over 10 minutes, and perhaps even infringing on her 5 minute preparation time.

Regardless, I believe your best bet in this case is to simply stand firm.  If she asks, tell her you and the students will be out ten minutes prior to her lecture start, and then stick to it aggressively so it's something she can count on.  She should, then, stop interrupting your activities and passive aggressively intruding on your after-lecture time. 

Another instructor is pushing me out of the classroom right after my class ends
Near the end of Star Trek V: The Final Frontier, McCoy asked the entity "Is this the voice of God?"  The entity responded "One voice, many faces," and a series of faces appeared onscreen, presumably depicting deities out of human history or mythology.  The faces seemed vaguely familiar, but I couldn't identify them.  Assuming these faces were taken from real source material, who were these gods?

Screenshots of the gods:





