A Suffolk County judge declined Wednesday to let prosecutors present testimony that a murder defendant had beaten and intimidated his girlfriend, ruling that such evidence is improper and that jurors already know she’s afraid of him.

The issue came up in Riverhead during the testimony of Noriella Santos, 27, who spent four days on the witness stand describing how Daniel Greenspan, 30, of Manhattan, had her lure another ex-boyfriend, Michael Sinclair, from Brooklyn to West Babylon.

Once Santos brought Sinclair there in the early hours of Jan. 31, 2009, she testified that Greenspan shot him to death.

Santos, who had been charged with second-degree murder in the case, cooperated against Greenspan and now expects a sentence of probation after pleading guilty to attempted second-degree robbery. Like Greenspan, she faced 25 years to life in prison if she’d been convicted of murder.

Prosecutors argued that jurors should hear why she did what he asked of her and did not turn against him for years.

Defense attorney Arthur Aidala of Manhattan urged state Supreme Court Justice William Condon to stick to his original ruling barring such testimony, calling it “extremely prejudicial.”

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During her days on the witness stand, Aidala noted that Santos has repeatedly “blurted some things out” about Greenspan beating her and berating her, but he hadn’t objected for fear of highlighting the issue even further for the jury.

“Noriella Santos’ fear is crystal clear,” Aidala said. “You can’t put the toothpaste back in the tube, your honor.”

Condon originally barred such testimony to prevent a jury from convicting him for crimes other than those with which he is charged. On Wednesday, the judge said there is no reason to change that ruling.

“She is a co-conspirator here. She’s not a victim in this case,” Condon told Aidala. “She was afraid of your client. That’s clear.”

But to permit further testimony of domestic violence would give prosecutors an unfair advantage by suggesting Greenspan has a propensity for violence, Condon said.

“To go further than we have would cloud the issue for this jury on whether the defendant is guilty or not guilty of this crime,” he said, referring to the murder charge.

When testimony resumed, Santos said during questioning by Assistant District Attorney Nancy Clifford that Greenspan got upset whenever she mentioned how good Sinclair had been to her.

“I felt like it was the only control I had in the relationship,” Santos said. She dated Sinclair when she and Greenspan were apart during the summer of 2008.

She testified earlier that she got genital herpes that summer and gave it to Greenspan when they reunited. Greenspan assumed the source was Sinclair, she said — and that’s why he sought revenge.

Santos has testified that Greenspan directed her to get back in touch with Sinclair, flirt with him and lure him to Long Island with the promise of a party. She said she believed Greenspan would rob and humiliate him, not kill him.