I have known several horses that you could give some hard kicks to and they have no problem sucking it up and staying there anyway. I also know some that will sit there as they are getting popped with the whip and it doesn't even phase them.
I personally have no problems using spurs on a horse that has become dead sided over time, however I would never encourage anyone to do so without knowing how to use spurs. Some people can wear spurs on a horse, use, pull away, or use forward cues without ever using the spur, in which case a spur can be useful on a horse like this since you can use the leg first, and then spur as a reinforcement to the aid similar to a whip.
I like to think about horses like this in their patterns. Gradually over time, that horse has learned to tolerate more and more pressure, so I want to reverse that. Consistency is very important, as well as the will to not give up. I personally will reward every effort forward by a horse that has "no go". Every step forward will be rewarded, not by cookies, but by stopping all pressure. This is sometimes easier to teach by whip on the ground, since the cue can then be transferred to the saddle. If you are consistent in your timing, release, pressure increases, and rewards, then the horse figures out very quickly that its much easier to move off the touch of the leg than it is to wait for the harder pressure that is sure to come. I won't lie, I've sat with the same horse for 20 minutes to get one forward step, which is where the will has to come in. Get a good drink ahead of time, and have it set in your mind that you are not stopping what you are doing until that horse takes a step forward. Your persistance and attention proves a lot more to the horse than you are more stubborn than they are. That same horse I spent 20 minutes with the first time now moves forward off the lightest touch or voice command. You don't have to beat the snot out of them, simply out last them. The longer the horse has been able to get away with not respecting forward aids, the longer it takes to get them to move.