Is there a way to limit the blocks that a player can place in a level? I don't want to stop people from placing blocks outright but I want to choose specific blocks that are allowed to be placed. In the system I'm imagining this could be done with an already existing world or level (.lvl file). Any help would be greatly appreciated.

First off, there might be a plugin for that - maybe ask around if someone else has one. Unfortunately, I don't have it myself, but if it doesn't exist yet, I'm sure someone will probably create it for you. Secondly, you could try setting it up directly on the server - I think the limit is server-side. I haven't tried it myself, but it seems possible, though it's kind of a 50/50 chance.

I might be the first one trying to help you out with this, but I definitely won't be the last. And if nothing else worked, you can always just put up a sign asking people not to place more blocks ;)