Charles Nagel, 36, from Philadelphia, Pennsylvania, is on trial at Brooklyn Federal Court charged with federal interstate stalking for allegedly sending the actress a series of strange love letters and turning up at the set of the TV show in New York.
He is also accused of targeting Erbe's teenage daughter by sending her messages on Facebook.com and doctoring a picture of the youngster.
Erbe took the stand on Wednesday and told the court she immediately distrusted Nagel on their first meeting, saying, "I got a very bad feeling from him. Just the way he looked at me... I was extremely uncomfortable. He stared at me openly."
And her misery was compounded when she found out he had allegedly been attempting to contact her through her daughter, and she was disgusted when she saw a doctored picture of the teen online.
Erbe sobbed as she said, "I couldn't believe it was happening... That is a seventh-grade graduation picture of my daughter with a dialogue bubble that says, 'I'm ugly,' and a cockroach drawn on her face. This is my daughter, who is completely innocent and relatively defenceless, and I'm not going to let this happen."
Defence lawyer Robert Datner then called Nagel's 12-year-old daughter Harley Quinn to testify she doctored the picture, not her father.
Nagel, who denies the charge, later took to the stand himself to claim he is just an over-zealous fan.
He told the court, "You have to be over-the-top to get a reply from a star. I try to be sweet and sometimes it doesn't pay off."
The case continues.