How to refute the claim that the Buddha was an "avatar" of Vishnu?
The easy way:

Ask for scriptural evidence for the claim. There is no reference in any Hindoo scripture as an evidence to this. 

The difficult way: 

Show that it is not possible using Buddhist ideas of Annata.

Easiest way:

Stay away from these arguments. I know where this is coming from. This is not disprovable or provable no matter how much you argue.