I've recently started as a PhD student, but had previous publications in forms of conference papers and various talks, so I consider myself very experienced with my particular field of research. One of my supervisors had once assigned me for a review task for a journal because she did not have time to complete it. This was fine with the editor of the journal, too.
Starting from that date, I began receiving invitations to review manuscripts every few months. Of course, I took that opportunity and completed the reviews – which naturally takes a lot of time.
Now, I cannot really invest the time anymore. I've previously rejected two review invitations with "I do not have the time to complete this review" (or similar), but I keep receiving invitations.
What could I do to politely ask the editor not to consider me for review?
Is there anything I'm about to do that could be considered inappropriate? I understand that reviewing activities are positive contributions to my academic career, but I don't think I can take it now.