Ok. possibly. But a CAPTCHA is not an option since it would be to annoying to the everyday user.
Since you see that shortcomings of applying a CAPTCHA (although it is was invented to do exactly what you need) outweight it’s possible advantages – you could try tackle a problem following two methods:

1. Think of what is typical bot/robot is going to do on YOUR web site. Then think of typical user scenarios. The key here is to grasp the difference - how the typical robot’s behavior could be different from the behavior of typical user of YOUR system.

And then re-design your app in a way to make navigation, page refreshing, etc more difficult to the visitors which behavior falls into “suspicious” category. This will likely introduce a problems to a small percentage of human users as well, but not to “typical-everyday” users.

2. You may also decide to delegate some of anti-robots functions to a human. Track the unusual behavior and report it to person who is looking after the web site (“moderator” or “admin”). If the reported user seems like a robot, moderator could decide to ban his session or login.

If the numbers of robot attacks are small and it’s alright to have some delay in reaction to these attacks - this might work for you.

Try mixing these methods.

As for your question - java applet will not make a big difference as it could be easily grabbed as an image and OCR-ed.