I disagree, as many folks are undergrad students needing help for their studies. Of course the people answering stuff will likely be grad students or PhDs, but we should definitely not reject questions from students. Rejecting this bulk of people would further degrade the number of visits and more importantly, the number of questions per day. If we ever wish to graduate, limiting the site to 'researchers' would be about the worst thing you can do.
Instead, we should close poorly researched questions. At Biology, and I have mentioned this in chat, there is the off-topic homework taghomework tag. Although the tag name itself has been debated hotly (it may be insulting to researchers), its use is widely accepted, namely to close-vote poorly-researched questions or trivial questions that are answered with a single Google-search and more suitable for quora or reddit.