Don't try to reason with the unreasonable. If someone is truly unreasonable...why bother? 

Tips

1. Stick to facts - it's too easy for them to attack your opinions 
2. Don't debate - you'll come away frustrated and de-energized  
3. Don't sell - they don't want your ideas
4. Minimize interactions - the less time you're around them the better 
5. Stay positive - they'll tend to find someone else to debate