I got 700 Newsletter subscribers per day for my site, the subscriber name and email id not relevant. What is the possible way to prevent. I think these are all spam accounts.
An additional method to asking confirmation from your subscribers is banning those IP's which are passing fake subscriptions for a period of time let's say 7 days. Use fail2ban which is a great tool, here is how you will identify a fake subscription in your webserver access log:
POST /newsletter/subscriber/new/ HTTP/1.1" 302 570 "h t t p://mydomain.com/" "Mozilla/5.0 (Windows NT 5.1; rv:7.0.1) Gecko/20100101 Firefox/7.0.1
It is clear to me this is a bot, in the last two days there are 200 lines into my log with the same content coming from different IP's. This is the part to use with fail2ban to create a new jail: POST /newsletter/subscriber/new/ HTTP/1.1" 302 570. Just grep your logs for this string and see what is happening :)
PS - stackexchange did not allow h t t p without spaces above.
My favorite solution is a free extension: https://github.com/magento-hackathon/HoneySpam
About Hackathon Magento Honey Spam
There is also a check if this form is too fast transmitted, a human normally takes a couple of Seconds or more.
You can enable and disable the functionalities or set the time how long it should take to transmit this forms in the Magento Admin Backend.
Freshly added: Regex-checking on all input-fields to determine a loose index for spam-level. It is not the most accurate thing at all, but it may prevent the low-end spam ("broadband spam"). You also can enable/disable this feature in the backend and set a maximum spam index trust level.