The way to reduce bad beliefs is to, IMHO:
1. Ingratiate and investigate. Respect the people, become their friend, listen to their full arguments (don't imagine straw men), understand their motivations. You might even discover the belief isn't so bad. If you don't, continue on to:
2. Suggest alternate ways of viewing the situation. People will always argue their point and appear to not accept your view. But I've found they almost always actually move slightly towards your point, oftentimes after the conversation, even if they won't admit it. It takes time for a worldview to shift.
Arguing makes no sense prior to ingratiating yourself, because if you are an enemy your view will not be appreciated.