There are a couple existing site settings for rate limits at /admin/site_settings/category/chat
Users who are flagged will also be silenced for a period of time:
And you can also limit which groups have access to chat (or to direct messages):
Finally, there’s this setting, which could be raised to 1.0 prevent the particular issue you shared:
(from a quick test, it works at 1.0, but I think there may be an issue where it has no effect if it is set to any value less than 1.0)