The only long term answer is to dissolve the spur. This can be achieved by dealing with two underlying reasons behind their development in the first place.

We need to improve the circulation and healing potential in the hoof generally and encourage the re-absorption of the spur which can be seen as an excess calcification (like a scar) on the bone itself.

My program with normalising bone growth by providing specific nutrients and healing herbs to be found on my website:
http://www.herbal-horsekeeping.com/horse_tendon_bone_healing.html will support the re-absorption of the spur itself.

This will not really work over any reasonable time frame without dramatically improving the circulation within the hoof that is achieved by using my external oil treatment see:

http://www.herbal-horsekeeping.com/horse_hoof_oil.html

Together these will quite rapidly begin to reverse the problem and you will see any lameness reduce as a sign that it is all working. If you are using Bute try not to use it for too long as the side effects can have a negative impact. A herbal anti-inflammatory maybe equally effective.