Plan A: We organize a national referendum. When millions turn out to vote for the resurrection of Seinfeld, Jerry will be so touched he will have to return to the air. It will be a lot of work to get the movement organized, but its doable.
Plan B: If Plan A fails, here's the backup: someone slips Jerry a mickey. Something mild that just induces him to take leave of his decision making capacity. Then we spirit him to Las Vegas or Atlantic City or Monte Carlo and get him into a high stakes poker game in which he loses hundreds of millions of dollars before he comes to his senses. Then he will be forced back to the screen because of his newfound desperate circumstances. While certainly an extreme measure, I think in this one case, the end justifies the means.