Zagan's held onto it for millennia and has been constantly upgrading and augmenting it. In between it being better than an Exorcist should be, and being filled to the brim with all manner of arcane machinery and archaeotech, and the fact that his preferred fighting style is "control other ships and have them do it for me", I'd say the only way to end the space battle at the moment is to withdraw beyond the range where he cares about your existence. He's much more interested in the planet anyway, and at the moment, the Hammer is an irritant at best.
It being an Exorcist, the ship has rather a lot of bombers to discourage any attempts to get close, to say nothing for its guns or the fact that Zagan can just enslave your systems.
There's a reason why he can do that, by the way, and before anyone asks what it is, all I will tell you is that he is XII Legion -- sorcery is a no-go.