Beltran wanted to quit in season 6 but wasn't allowed to do it.
I don't understand why they didn't let him quit. He was bad-mouthing them at every opportunity and his character wasn't exactly doing much or bringing anything to the series.
If they'd gotten rid of him then they'd have brought back the skant when Seven was somehow made the ship's new XO, though this one would also be ridiculously low-cut as well.
if they wrote him off then Tuvok would be XO
as for not letting him go, I think it has to do with the contract