Chuck D tells TMZ Sports ... if fans upset with the Colin Kaepernick situation REALLY wanna send a message to the NFL, the best way to do it is with a full, serious boycott. 

"Don't go to the sports bars. Don't turn the game on. Leave your team for a year."

The Public Enemy legend says it's a bunch of crap that Kaep hasn't been signed to a team -- claiming his accomplishments on the field speak for themselves. 

Chuck says he thinks it'll be tough for protesting fans to really make a difference -- but if you are gonna protest, go all the way. 

But does Chuck really think people will commit to cutting off football? Probably not.