Unfortunately the National Labor Relations Board can't do much about it, reports Michael Whitney at firedoglake. The NLRB is charged with enforcing labor law, but
...this responsibility comes almost no enforcement power for the NLRB to effectively hold accountable corporations that break the law.
All the NLRB could do was require the owner to post a notice that said:
“We will not threaten to kill you or to cause you bodily harm because you engage in activities in support of SEIU, Local 32BJ."
Here's the whole story from Newsday.