I could have sworn this was already a dedicated thread, but my google-foo failed me. I am interested in how others would handle this, as it always happens away from the FAs.

Twice this summer I've had to get someone out of my assigned seat. Both times, it was an MCE. Once, my seat was paid for a week before, and the other was a seat change at check in. Both times the other person said something like, "Oh I am in seat so-and-so" and moved, without actually checking their boarding pass, to another MCE seat. Seriously? Both times, their new seat was on the other side of the aisle in a different row. Both times the person was a solo traveler, so nothing like a "please switch with me" scenario.

Out of curiosity, during boarding, how would you handle this if the person didn't move so easily? Physically stand your ground and block the aisle? Attempt to get a FA's attention? Try to swim upstream back to the FAs?