Josef, as far as I know there is no such existing plugins that will limit the amount of messages.

To protect your site against spammers we can recommend you the Watchdog plugin.
Also we could suggest you to enable the mandatory user approve functionality.
Go to www.yoursite.com/admin/permissions check the box Mandatory user approve.

And this topic will be useful for you: http://www.oxwall.org/forum/topic/11284

Thanks Taissa 

All is set as you suggested , the problem is i approve the user but then i discover they are sending  spam emails , they are real persons not bots !

Thanks 

you can use user credits to limit mails or anything else you like

human spamers theres nothing you can do to stop them you just have to find them and remove and ban them or hope members report them as spammers 

Josef, as Tammy have suggested you might try using the User Credits plugin.
 
You can find more information about the User Credits plugin here: http://www.oxwall.org/forum/topic/23395

In case this solution will not help you, the best way is to develop the plugin. You can find a third-party developer at http://www.oxwall.org/market/specialists .

WebFid Solutions, please, could you clarify how the deleting of the captcha code could change the situation with spammers.

i was getting hammered so bad by bots n stuff 
advanced captch plugin worked fo a wile but they found ways around it
i use the google captch plugin api thing maybe 3 or 4 months now and is great 
has stoped the pesky buggers