What can I do if I am constantly invited as a reviewer, but don't have the time?
As far as I know, there is no official please-do-not-bother-me-ever-again button that removes you from the reviewer list in a journal's submission system. That being said, I assume just writing the editor a polite email should do the trick. Other than that, you can always continue to refuse reviewer invitations, and I am pretty sure editors will relatively quickly stop sending you invites (they hate wasting their time, too).

That being said, I would personally urge you to reconsider your stance on this. Reviews are an important community service that are essential to the functioning of science, and one review every few months really is not an over-the-top amount. For instance, I review at the very least one journal submission every month, plus proceedings papers for conferences and workshops where I am in the Technical Program Committee. That you recently started your PhD studies should actually only be more of a motivation to keep reviewing, as this is actually a pretty good way to stay on top of your field.