It totally depends on many factors, as to whether the act is considered illegal.
If person A, expresses dislike for person B, using true statements (if evidence of such truth is made available upon request (but remember, laws regarding privacy exist too)), then it's fine, but if the expression is often, it could be considered harassment.
If person A, expresses dislike for person B, with falsehoods, then it could be considered an act of "Slander", and if person B wishes, could argue that you also created a "Defamation" of their character, which could be argued, that you also affected their business, not only making you applicable to certain Fines, but also charged for recompense of any business earnings lost, during the period of defamation, for the working days of person B.
For person A, it's smarter to just ignore someone they don't like, because, everyone dislikes someone.
For person B, it's smarter to respond to person A in a calm manner, requesting that person A's actions desist (stop), and if, person A, fails to desist acts of harassment and offense, then person B should pursue the matters outside of the public forum, and continue the discussion perhaps with a mediator (if a previous relationship existed), else, desist all direct communication, and record all acts from person A, towards person B.
And then, if necessary, take legal action, to protect their self.