Daniel Morris lost his temper on the footbridge, near Burger King, on May 8, and squeezed the 16-year-old girl’s arm when she threatened to call police.
“He said - “If I get sent down I will come back and finish the job,”” said prosecutor Donna Fawcett.
And when the 16-year-old girl tried to leave on her skateboard he kicked it away.
The girl said in a statement that their relationship had recently broken down because he had been “grabbing me and ragging me around”, and she believed that “no one has the right to assault me.”
Morris, 25, of Welbeck Street, Mansfield, admitted assault when he appeared at Mansfield Magistrates Court, on Wednesday.
Donna Pursglove, mitigating, said the “very minor assault” had been sparked when the girl told him: “If I am pregnant you will be the last to know” and she had threatened to put any child up for adoption.
She said Morris was a railway engineer on a zero hour contract, who received universal credit and was also training to be a brick-layer.
He suffers from “severe” attention deficit hyperactivity disorder (ADHD) and had been under a lot of stress, Ms Pursglove said, because he had been coping with the death of his aunt, and a family illness.
Morris was banned from contacting the girl or going to her address for 12 months.
Magistrates fined him £40 with a £30 victim surcharge and £85 costs.