Mirabelli Stefania pled no contest to one count of misdemeanor aggravated trespass and the judge immediately sentenced her to jail, but not a lot. She got 8 days, but only has to serve 4. She's been in jail since her arrest earlier this week so she'll be back on the streets within a day.

Stefania will be on probation for 12 months, and she's been ordered to stay 100 yards clear of J.B.

TMZ broke the story ... Stefania was busted at Justin's Bev Hills pad after cops say she wandered onto the property for the third time in a week. Bieber was home at the time but she never made it inside the residence and he never made contact with her.

Here's the problem ... obsessed fans are the least likely to obey stay away orders because they're often not operating rationally.