TB54 that's a great idea for an extension, but it's a bit out of scope for this extension.
Even if the picker library I used here allowed to filter down the available emojis (which I don't think it does), this would still be a client-side only feature.
Properly limiting post content for abuse requires server-side code that hooks into the formatter to change rules. And deciding what is an emoji and what is a valid character could actually be quite complex because it's all just Unicode points behind the scenes.
I feel like this would be best handled by just publishing clear community guidelines and taking action against infringing posts.
If you really wanted an easy way to block too many emojis automatically, you could use custom CSS that targets the image tags inserted by Flarum Emoji extension, and use the nth- selectors to hide repeating emojis or specific emojis by their src image URL.
Maybe the existing AutoModerator or Filter extensions could already be used to automatically flag or redact posts that contain emojis matched from a regular expression.
