[Mailman-Users] Removing archived spam

[Hal]

I run a low-volume mailing list (using Mailman 2.1.12) and I see that a 
few spam-messages have gotten through, which also means they're archived.
I would like to remove them but all the info I can find when searching 
online are along the lines of "hard to do", "shouldn't be attempted", 
"impossible" and so on. Is this correct, or is there a solution?

Perhaps it helps to say that for the past two months (September and 
October) each of those only contains one message each, and they're spam. 
Other months contain a mix of spam and legitimate messages.

Hal