... and it makes me suspect that your group is a bit of a powder keg. I think you should probably try to fix whatever makes it so easy for sessions to dissolve into rage. Otherwise you might be facing this kind of problem again in two months, over some other random detail, possibly in a different system and setting. (Unless this is really an anomaly for your table. I don't know your players, obviously.)
This, this, a thousand times this!
They may have seemed reasonable in other contexts, but they went nuclear over this issue. WHY?
Did they know the DF stories? How well?
The thing is, there was clearly a mismatch of expectations & understanding of what a circle is/does; a wizard, in-character,
would know what a circle is/does, and
wouldn't raise a circle expecting it to last in these circumstances.
Letting a PC do something in the clear
player-expectation of ONE outcome, when the
character would clearly expect something ELSE ... well, yeah. I'd cry foul, too.
If I'm GM'ing a game where "draw a magic circle of protection" ONLY protects against magic, AND can easily be mundanely-broken, AND there are Mundane combatants around to break it ... well, I'd feel
obligated compelled :-] to offer the player a caution, "Your character would know that this circle can easily be broken by THIS or THAT combatant, if they know they SHOULD break it."
OTOH, if the player was familiar with the stories, then I'd presume they already knew this, and were doing some tactical/strategic thing (and wouldn't bother with the caution).
Given this level of misunderstanding, I'd do a serious run-through of game mechanics and setting-expectations.