Thanks for your input, but is there anything I can do at this point?
Write a brief, clear and coherent email and send it to United Customer Service.
Stick to the facts and don't dwell too much on your feelings.
Say that you were prepared to repack and throw out the extra one pound, but the rep was being the opposite of helpful and prevented you from doing so.
If you have done a lot of flying with United, mention that as well, in a matter-of-fact way.
You've got nothing to lose...