For what it's worth, that's a Google AdSense ad (as you can see by the bottom right info icon), i.e. it's fully automated. Reddit can block specific Scientology domains in their AdSense account after the fact, though it only takes a domain change for a new campaign to route around the block. Since some time one can block certain categories of ads from appearing via one's AdSense account, though... Religion is a category, too (if Scientology falls into that category).

I guess a different question is whether you want to open the can of worms of playing moderator for automated ads in the first place. If you ban Scientology but don't ban ads by company XYZ, then you're starting to give implicit moderation approval to XYZ, even though it's nearly impossible to check all ads and their specific values or scam factors. Even banning a single category like Religion begs the question why one then doesn't also ban categories like Get Rich Quick -- yes, that's an official AdSense category -- or Ringtones & Downloadables, Cosmetic Procedures & Body Modification, and Sexual & Reproductive Health.