I assume that you must be using a stabilized chlorine as your sanitizer for the spa. The stabilizer in the Chlorine is the Cyanuric Acid. Each time you add chlorine to your spa, you are also adding CYA to it as well. After so much is added it just builds up more and more, and that is why it is reading high on your strips.
There is nothing you can add to lower it, other than fresh water. So in your case, I would drain and refill. But, if the water is clear and smelling good, you should still be in good shape until your next drain and refill. High CYA on pool can cause lots of problems, so I assume if too high on a spa, it can do the same as well. Here is a blurb about CYA that I copy and pasted off of the net:
The second (and more significant problem) with a high stabilizer level is that it greatly reduces the effectiveness of the chlorine in the pool. Remember that stabilizer works by slowing down the reaction of the chlorine in the pool. At higher concentrations, it slows down the reaction so greatly that it significantly reduces its effectiveness.
This is seen in the fact that yellow algae becomes a lot more difficult to control and the water can become dull. The unseen hazard is that it is also less effective against bacteria, etc, leading to concerns about how sanitized the water may be at any particular moment. The kill time of chlorine is normally measured in milliseconds, but at high stabilizer levels, the kill time becomes seconds and minutes, and time is critical in water sanitization.
Hope this helps! 🙂