A Penn State fan ain't apologizing for a letter he sent to a football player that many found offensive ... in fact, the guy is DOUBLING DOWN on it, saying, "Get the guys cleaned up."

In case you missed it ... a man named Dave Petersen wrote a letter to Nittany Lions team captain Jonathan Sutherland -- begging him to cut his "awful" hair.

"Watching the Idaho game on TV we couldn't help but notice your -- well -- awful hair," Petersen wrote ...

"Don't you have parents or girlfriend who've told you those shoulder length dreadlocks look disgusting and are certainly not attractive."

Sutherland's teammate tweeted out a picture of the letter ... writing, "Explain to me how this isn't racist."