Here is a list of sponsors from today's Broncos game. While virtually impossible to boycott them all, you can write them on social media with something along these lines:
This American is boycotting your products as long as you support the NFL that allows employees to disrespect the national anthem.

Southwest
Geico
Verizon
Taco bell
Buffalo wild wings
Energizer
Pizzahut
Car gurus
Apple
Hyundai
Progressive
Sonic
Home depot
Pepsi
Priceline
Mercedes
Modelo
Chantix
Snickers
Dickies
Corona
Dominoes
Lowes
Jaguar
AARP
Cricket
Quicken loans
Bud light
Smashburger
Autonation
Old spice
Sony
FedEx
Toyota
Lincoln