The forum should take a look at Google's new reCaptcha v3 https://developers.google.com/recaptcha/docs/v3

There isn't much information I could find on in it but it looks like it is designed for this sort of job. You define certain actions and then give them a score of 0-10 for good or bad then it uses the total score to define if something is a bot. So I think if you define posting and then deleting immediately as bad action and it should take care of bump bots.