To people suggesting trades shouldn't be possible during combat, let's not throw the baby out with the bathwater. There's legitimate reasons to be able to trade with people in combat ("I forgot my food/meds, and we already started! Cans someone trade me a few? Thanks!").

The real issue is that the trade window interrupts actions. That is what should be corrected, not the ability to trade at all. If someone trades you, it should not interrupt your gameplay UNLESS you choose to accept the trade. FFXI did this well; each player had a "Trade" command, and if one person traded to another, absolutely nothing happened apart from a "Xxxx wishes to trade with you" message. The only way to accept the trade was to activate your own trade command in response. (It also had a cooldown period during which you can't initiate another trade if the first trade is refused.)

If I recall, there's a similar issue with RMT and friend requests, with RMT sending a friend request in order to prevent you from blacklisting them before they have a chance to log the character out. Really, popups of any kind should not be allowed to interfere with gameplay. In the OP's case, there should be the option to just ignore the popup and continue as usual, with the offender unable to initiate any further trades because the first was neither accepted nor refused.