Lately, I am getting calls to the newsletter subscribe module from outside servers. Is there any way to stop it or force Magento to validate the call is from the server?
I recently had the same issues. We fixed it by adding this module. https://github.com/magento-hackathon/HoneySpam Basically it adds a hidden field to some of the Magento forms that often get abused by spammer. When the hidden field is filled (what spambots do) the request gets blocked.
If the requests really come from an outside servers directly to the controller you could also add formkeys to the newsletter submit form and check for them either by overriding the controller or with an observer. If you don't know how to do this let me know and i can provide with an example.