Spam thru :"tell a friend" button

Hi,

I don't know how but two days back, my site started generating lot ( really a lot @ 100 mails per 20 mins approx) of spam.
I have enabled the option where i get a copy of mails sent thru my site. Hence I came to know that these mails are being generated using "tell a friend" option.
By the time I realised spam is happening my mail box had over 4000 mails!
I contacted my hosting company(Hostgator) and they added TXT records and after 4-5 hours the mails stopped coming.
I still don't know how all this happened & how to prevent such instances?
Also worried about that without my knowledge some 4000 people would have got spam from my company. Not a very good thing for a startup like mine!
Any advice is welcome.
Thanks
Devyani.

Hi All,
I am not sure whether this is the right forum to post this message, maybe that's why I am not getting any reply.
The spam thru "tell a friend" script started again, and with this time the rate of spam was higher ( @ 2388 messages in 3 hours !)
As of now the hostgator guys have disabled the script.
But this is not a permanent solution.
Will putting a captcha help?
Pls advise
Thanks
Devyani

One of many similar threads: https://www.zen-cart.com/showthread.php?200475-100-s-of-SPAM-emails-per-day
FAQ article with suggestions: http://www.zen-cart.com/content.php?92-how-do-i-turn-off-reviews-and-or-tell-a-friend
Or, you could try captcha if you wish, or a CSS-based anti-spam approach also discussed in the forums.

Thanks Dr. Byte. I have done this.
But tell a friend is a good feature by which my products get recommended and site gets publicity.
Is there any other option, if i do not want to disbale this feature ?

I have installed captcha and enabled the tell a friend link.
Things are moving smooth now.
Thanks for all the help