Treading the digital domains for research can be somewhat tricky for students and professors alike. Some of us have to bridge a generational or technological gap to even communicate with one another about our sources.
I had some recent success the past few years by embracing a discussion with my students to help us all communicate our expectations before (and often instead of) noting something is "off limits" for their research. Instead of outright banning a type of reference (i.e. no Internet sources or no Wikipedia), we dialogue about research together. We reach the conclusion (again, together) that a variety of references from a variety of source types creates the strongest argument. I show the institutional library page, we do Google and Wikipedia searches, we dissect sources, we really engage with issues of accuracy, objectivity, and authority. While we explore all of this, we discuss and analyze. Different classes come to different conclusions about wikis and blogs, but the (often lively) discussions allow students to consider and explore digital references in a way that is open without simply banning some source with an off-handed comment. I found students did not know why professors banned certain sources or types of references. Many of our students today have only and always found sources on the Internet. We might need to reach out and explore together a variety of considerations with online sources while introducing and exploring a variety of research avenues with our students.
The class time it takes to do this is certainly repaid in the quality of research received in the projects the students submit.
When you introduce that big project or the term research paper, consider a few simple tactics. Ask your students what they use, why, and begin a discussion while introducing concepts about research, credibility, and audience. Open the door to discuss by avoiding an initial ban. Of course, you can always ban a single type of source at the end of the discussion if you so desire, but at least the students will understand why you are doing it and have other options and better research skills because of the discussion.
You might be surprised by what can happen when you listen to the students when exploring research, I know I have been.