Deal with the ceiling first, because it is the thing you actually asked for and it is separate from reducing the traffic.
The uncomfortable fact is that most usage-priced platforms do not offer a true hard cap. They offer budget alerts, which tell you after the fact, and people discover the difference exactly the way you did. So the ceiling usually has to be built rather than switched on:
Set the alerts anyway, at a low threshold, and make sure they reach something you look at within hours rather than an email address you check weekly. Alerting at 50% of what you would tolerate gives you a chance to act while the number is still small.
Find out whether your platform has a spend limit that actually stops service, and if it does, turn it on and accept that hitting it means the site goes down. For a side project, down is better than an unbounded bill, and that is a legitimate choice you should make deliberately rather than by default.
Put something in front that has its own free ceiling. A CDN or proxy layer that absorbs requests before they reach the metered thing changes the shape of the problem entirely, because the expensive resource is no longer the one being hit.
And the general rule this is teaching: on a personal project, prefer a plan with a fixed price and a hard stop to one that scales smoothly. Smooth scaling is a feature for a business with revenue per request and a liability for a hobby with none.