The administration can just keep coming at him as long as they're in power, and that is itself effectively a punishment. If each case takes just 90 days to play out, they can bring four or so of those per year, on the taxpayer dime (while themselves getting paid, by us, to do it, in fact!) and waste tons of Comey's money and time, while also stressing him out.

The safeguard against this is supposed to be that Congress would eventually put a stop to it, or that the people wouldn't vote someone in who'd abuse the power of the executive branch to extrajudicially punish opponents. Neither of those safeguards have worked. Courts can tell them to stop but they have to keep telling them with each case, after everyone goes through all the motions (so to speak).