From my pov the easiest way, w/o writing a single line of code about your custom restriction ?
Just add the ID of skill into _heroSkillsId list @ SkillTable.java
These skills are 'removed' when you enter oly and then they get back :P
Or maybe not, since when the char is hero and he lose the status, he will lose the skill as well.. So, not rly a good way :troll:
But well, you could use the system how these skills are deleted, I guess. Create new method like is in SkillTable and add a proper check @ AbstractOlympiadGame about the removal. Well..
Fast idea, mby not the best, but still :D