An off-duty border guard tricked a prostitute in Thessaloniki into offering her services free of charge in exchange for a false promise to give her police protection, authorities said yesterday. Police said the border guard had set up a meeting with the woman over the Internet at a central Thessaloniki hotel and threatened to arrest her as part of a crackdown on illegal prostitution. Fearing arrest, the woman met his sexual demands, only to be arrested shortly afterward as part of a real vice squad operation. The women informed authorities of what she thought was an agreement to avoid arrest and police tracked down the border guard. The suspect will face charges of blackmail in a Thessaloniki misdemeanors court.