It might be impossible to fire them. But it’s not impossible to assign them a specific task. It’s not impossible to tell them the case they are working on is dropped. You may note able to fire them, but you can supervise the living hell out of them.
Put it in their permanent record that they engaged in partisan activities. Then fire them for cause.
Even if it was illegal to fire them, it would be worth paying any fines if it got them out of the office. Everyday the aren’t able to wield federal powers means another day for non-partisan prosecutors to drop cases and undo Obama level corruptness. An hour, a day, or a year delay means any shenanigans those prosecutors were secretly up to might come completely undone.