Simple fix: Remove all implications from “spoiler:s08e14” and “spoiler:s08e18”. Put together a simple script to add “spoiler:s08e14” and “spoiler:s08e18” to every filter in the booru’s database, including everyone’s personal filters. (Optional: Remove them only from the global “Everything” filter.) Have each tag imply the other so that every image showing either episode becomes tagged for both spoiler14 and spoiler18.
Remove the implication. All users will see all images from both episodes spoilered. First user to look at each image can remove the irrelevant tag and leave only the correct (14 for A Matter of Principals; 18 for Yakity Sax) one behind. Users who don’t care, or who have seen both episodes, can remove the spoilers from their own filters whenever they want to. Mods can remove them from the global filters whenever they feel like it. I doubt it will take a day for all the images from both episodes to be tagged correctly.