Simple answers, though I am not the first in this thread to give them:
- Abuse of the economy is RMT.
- Abuse of the economy may also be the use of gold gained nefariously by scams (actioned additionally and separately) or botting (actioned additionally and separately).
- Someone whispering you talking about undercutting you isn’t against rules.
- The actual act of undercutting isn’t against rules.
No, you cannot get this person banned, even though they really upset you with their whisper(s). Ignore them. Avail yourself of the tools meant to help you ignore people, if you find it difficult just to mentally ignore their behavior.