Students not answering emails about plagiarism
First, some important caveats:

It is certainly worth checking their other work to see if you find any further evidence of plagiarism. In the absence of such evidence, you must proceed under the assumption that the only case of plagiarism is the one you have evidence for.
You should check your university's regulations; you may be required to proceed in a certain way (e.g., to advise the students of their rights, or to turn this over to a committee rather than handling it yourself).
Since you are a PhD student, speaking with a more experienced professor is probably also a very good idea. In particular, if you are not listed as the &quot;instructor of record,&quot; you should speak with the instructor of record before proceeding (different universities will have different policies about whether the grad student who does the &quot;actual work&quot; will be formally listed as the instructor of record).

Still, let us assume for the sake of this question that you can and must handle this yourself. In this case, my advice would be to send an e-mail along the following lines:

Dear Student: it has been over a week, and I have received no response to my below inquiry. If I do not receive a response by Friday, I will assume that your report was indeed plagiarized and will accordingly assign [some appropriate but relatively harsh penalty]. If you believe you are not guilty, or if you have mitigating factors you would like me to consider, please let me know, either by e-mail or by scheduling a virtual meeting. Regards,

As for what penalty would be appropriate, I reiterate my advice to discuss this with someone locally; campus cultures vary, and your department probably faces issues like this with some regularity. Still, my experience is that for something like this, a proportionate response would be to (1) give a zero on the lab report in question, which should cost them about a letter grade overall, and (2) file a report with the college so that they cannot have a new &quot;first offense&quot; in a different class.