[newfeatureok]

You are comparing apples to oranges. Again, are you imposing a hard constraint on the number of comments someone can make?

With the example o gave you could have a constraint in CouchDB achieve the effect but there are simply other examples one could use.

[yatsyk]

No I'm not assuming constraint on the number of comments. First example shows how easy limit number created objects. Spam prevention is other topic not so trivial but mostly solved problem.