How do you announce a forced sign-up wall without sounding corporate?

We finally had to kill our anonymous, no-account-required tool because bot traffic was quietly draining our API budget. Now, anyone who wants the core feature has to make a free account first. The issue is that the people losing the friction-free experience are anonymous visitors we have zero contact info for so our landing page announcement feels like shouting into the void.

If you've ever gated a previously open utility, how did you explain the change without sounding defensive?