I've noticed that more and more websites are disabling the ability to zoom on an ipad using Safari (or other mobile browsers, it seems).
Is there a setting to turn this ability off, or some other way to prevent this?
There is not a way to prevent this using Mobile Safari. However, many alternative browsers will let you change the user agent sent to the server, which means you can make it look like you're connecting with a desktop browser instead of a mobile browser. While this will work, I understand it may not be exactly what you're looking for - unfortunately, you can't change the default browser without jailbreaking.
There is not a way to stop a website from disabling zoom in mobile Safari. The zoom is disabled in the HTML code like this:
To disable this, you would have to have an HTML stripper that removed this HTML from the webpage. You might be able to use a Cydia app, but I haven't found anything that does this.
Your best bet is probably to use an iOS browser that allows you to change the user agent. Most websites insert the HTML above when they detect that your browser is a mobile browser. Some free web browsers include:
A more complete list can be found here.