Exercise induced hives. What can I do about it?

Very little. Sometimes taking antihistamines before exercise will prevent the hives, but most people simply have to find the level of exercise that produces hives and stop before they get there. Sometimes these hives occur only if eating certain foods before exercise, and find that the hives won't break out if they don't eat that food within a few hours of exercise. Your allergist can help sort this out.
Consider homeopathy. Urticaria can be helped with acute homeopathic remedies; such as apis mel.