More often than not, I eat my lunch at my desk. There aren't any shops or cafés nearby, so this suits me just fine. I usually put my headphones in and browse the news or SE to take a break from work.
Occasionally, colleagues will come over to discuss/ask about something we've been working on, despite there being a meal on my table and zero evidence of work on my screen. These queries normally take a couple of minutes (if that) to discuss, and wouldn't usually bother me.
Today however, I've had two people coming back and forth with work requests for the past 20 minutes; asking me to open various documents and make amendments then and there while they stood over my shoulder. As a result, my lunch has gone cold and is inedible. It seems like common courtesy to me to not disrupt somebody who is evidently on their lunch break. How can I interrupt a colleague, and stop them from discussing work with me until after my lunch break without being rude?