Zack Hample ... the guy who caught A-Rod's 3,000th hit ... tells TMZ Sports that since snagging the lucky ball he's been dogged by people making "scary" threats for him to return the ball to the Yankees stud.
Hample ... a notorious ball hawk who actually writes a blog on how to catch dingers ... says some fans want him to return the ball so badly that they are filling up his inboxes with dangerous talk.
"I try to disregard it, but when some people make personal threats and say things, it's scary."
Zack also tells us he adamantly refutes the rumors that he knocked over kids and old ladies to get the ball ... saying Internet haters are spreading the lies to smear his name.
"What I do, is totally respectful, and done with the utmost care. It's all lies."
Hample still hasn't made up his mind about what to do with the rock just yet -- despite yet another offer from the Yankees -- but he says he doesn't understand why people are giving him a hard time.
"No one has a problem with A-Rod making like $20 or 30 million a year, but when some dude catches a baseball he's supposed to just hand it over to the rich guy."