The problem here is that /r/nocontext has essentially become a funny one-liner in bigger subs like /r/AskReddit. When people reply "/r/nocontext" as a joke to a funny post, most of the time they don't even think of posting to this sub--they just reply for the karma.

The way to improve content on this sub is simply to get more people to actually post to it and contribute instead of using it as a catchy hashtag.

As to the proposal itself: I think the worst thing you could do is start deleting a lot of posts. When mods have done this in the past, they've been crucified by the Reddit community. What would be better in my opinion would be a cautious, gradual implementation of this idea supported by efforts to get better content:

Start by establishing very clear, numbered rules for the kind of content that is expected in the sub. Then, start deleting a small number of the worst posts every day and give clear reasons that refer to the sub's rules as to why it was done. Gauge feedback from redditors and scale the project up or down as a response. At the same time, try and improve the content that actually gets posted, by encouraging people to contribute more and by making sure that when someone says "/r/nocontext", it actually gets posted here (maybe through the automated process you guys are looking into).

But seriously, the worst thing to do would be to suddenly start deleting large amounts of content from the sub. Implementation of moderation schemes like these has to be gradual and has to keep user feedback in mind at all times. Most importantly (and you guys are doing a great job so far), always be accountable and clear to users.