Cities Protected by Unfinished Military HQ

[QuakeIV]

I am unable to attack a city because of an unfinished military HQ building, which both provides invulnerability and also prevents flags from being planted, but also cannot be destroyed (or completed, even, which I also tried).

I could understand if the game was just vaguely biased against warfare or something, but this is getting ridiculous.  We finally catch this guy online and he is still invincible.

edit:
An associate suggested C4, no effect other than blowing big craters into the construction site.