In short:
Person A harrasses person B if they engage in unwanted conduct related to a relevant protected characteristic, and it has the purpose or effect of violating B's dignity, or creating an intimidating, hostile, degrading, humiliating or offensive environment for B. Also, if A engages in unwanted conduct of a sexual nature toward B with the same purpose or effect.
If this is happening, the company should have a grievance procedure, outlined in the staff hand book, so follow that, if not, they should follow the ACAS one.