Justin Bieber -- He'll Be Dogged Indefinitely By U.S. Customs

Justin Bieber's days of breezing in and out of the U.S. are over ... because we're told he will continue to be a target of U.S. Customs every time he comes back to the states.

TMZ broke the story ... when Justin flew into a New Jersey airport from Canada last Friday ... U.S. Customs searched his plane and then grilled him for hours, before letting him leave. Law enforcement sources tell us ... Custom's flagged Bieber because he's on a Homeland Security watch list for his 2 ongoing criminal cases.

We're told Bieber is subject to the exact same treatment every time he leaves and then re-enters the U.S. -- a thorough search and then exhaustive questioning.

It's very hard to get off the watch list once you're on it. You can appeal to Homeland Security but the agency has broad discretion. What's more ... if Bieber is found guilty or pleads no contest to any criminal charge, it will be extremely difficult for him to go under the radar again.