Maximum Emails per Hour

Users have disabilities so captcha will make it harder for them. Its new users who are spamming to scam users. On Joomla 3 the new users couldnt send out messages for the first 24 hours, as this gave me time to check the profile. I can spot a scammer straight away. I could approve the profile manaully,

Live chat and PMS sounds good, a bit like facebook.

You could use CB Auto Actions and a Redirect action that checks against their registration date on pm_onBeforeDisplayMessageEdit if they just newly registered. Example as follows.

Global
Triggers: pm_onBeforeDisplayMessageEdit
Type: Redirect
Access: Everybody
Conditions
Field: Custom > Value
Custom Value: [cb:parse function="time" time="+24 HOURS"][registerDate][/cb:parse]
Features: Format Functions
Operator: Greater Than
Value: [cb:parse function="time"]now[/cb:parse]
Features: Format Functions
Action
URL: index.php?option=com_comprofiler&view=pluginclass&plugin=pms.mypmspro&action=messages
Message:
That should work. It should redirect them back to the messages page if they've been registered for less than 24 hours. Adjust further as needed. If you need to manually verify accounts though I suggest just using registration approval.

As for captcha you could still consider using reCaptcha Invisible or reCaptcha v3 which typically don't require any input from the user, but should catch bots. The internal captcha mode is also completely accessible with the appropriate aria labels, customization to increase font sizes as needed, and audio. You're going to have a hard time combating spam without some form of captcha. At the very least I recommend having captcha on registration.

You are a star.. It works, thank you so much....

Yes I do use reCaptcha on the sign up form. Only 5 big letters. But does the Invisible reCaptcha do the same thing?

I just looked at the settings, its asking for

Site key and secret key. Where do I get these from?

Thank you
Karl

Yes I do use reCaptcha on the sign up form. Only 5 big letters. But does the Invisible reCaptcha do the same thing?

Invisible captcha doesn't ask them to fill out anything unless they're suspected to be bots. Then it presents image selection captcha.All re

Site key and secret key. Where do I get these from?

All reCaptcha keys are obtained from Google directly .

I have done Invisible captcha on the sign up form. This will be useful for users with learning problems

Is it possible to hide Invisible captcha on the form?

Thank you
Karl 

Is it possible to hide Invisible captcha on the form?

Yup, within CB AntiSpam > Parameters > Captcha > Modes > reCaptcha > Invisible set "Badge" to "None".