How can you configure a said to policy to prevent users from wiping their mobiles from within OWA?
Not entirely sure this is possible based on this forum post. Overall it looks like any user who has Exchange ActiveSync enabled has the ability to wipe their own device. The post above was from back in July, there may have been a feature update between then and now but I'm not 100% sure.
Is there a specific reason you don't want your users wiping devices? It seems pretty self explanatory as to what the feature does. Are you using specific software that cannot easily be reloaded onto the device?