I don't get why people leave Castrum. The way I see it, the primary draw of the MSQ roulette is to get poetics, and Castrum is nearly twice as fast as Prae yet gives just as many poetics. And if you're there for EXP on the side, you're really just shooting your own EXP in the foot by leaving and giving yourself a lockout.

That being said:

Make it so that at least 15 minutes passes before a player leaves a dungeon willingly thereby discouraging a player from leaving and that have to stay and support their party instead of continously abandoning them and making them either wait for a replacement or leaving the dungeon and forcing them to requeue, in DPS instances for a long time.
Do this and people will just d/c themselves or "afk" there waiting/asking to be kicked.

Ultimately you're still not gonna be able to force people to run something they don't wanna run.

I've seen people leave from both Castrum and Prae but rarely has it led to a totally abandoned run. The instances are so easy that a light party could still comfortably do them, so usually groups just soldier on and replacements usually come fast.