Identify them.
I love me my bitcoin. I sold some and gave away some (too soon!) but I have some left. I have $3500 worth on the laptop I am typing on. Amazing!
U.S. banks will report anyone taking more than $3,000 in cash as engaging in a suspicious activity using Treasury Form SAR (Suspicious Activity Report).
Visa is offering certain merchants a $10,000 reward if they refuse to accept cash in the future.