> Do I have access to their sats?

Yes, but there’s no standard UI for accessing individuals’ amounts. It’s in a Redis database that you can query. Presumably one could write code to update this database directly.

> Are they somehow separated?

LndHUB accounts are custodial entries in a Redis database. The sats are commingled in one shared Lightning node’s channels. It’s up to you to maintain enough liquidity to satisfy demand of your users.

> Can I rug them?

Absolutely. It is trivial to do so en masse. Just close all your channels and walk away.

Rugging an individual user is more difficult because there’s no UI for it. But the code is open source so developing a script ought to be possible.