Well you should delete unmarked spoilers, even with old series. If I were to search for some kickass anime (even if old and I haven't watched it) and I would see something that would have impact on the anime I want to watch, for example a whole new model in gundam that reveals itself later in the story, then yes it would be a disappointment.

Then again if this occurs quite a lot, you could try making a thread under Anonymous in /a/ asking people to mark spoiled content. If it's getting quite frustrating (Excessive threads about spoilers for example) I would suggest talking to one of the moderators, deleting threads over and over will get frustrated by janitors and anonymous alike, letting a mod know about it, and making a thread there stickied about marking spoilers would surely solve the problem :)