There are things that the local police can do.
Drive up to these persons who are hanging around the station and ask them kindly what they are doing there. Then ask the station attendant if that person is allowed to be on the property. If the attendant says no, then the officer can kindly ask that person to leave. They are in fact trespassing on private property (its not owned by the city) so trespassing might apply.
The police can detain persons for short periods of time for suspicious activity to check identity. I would say that hanging around a gas station at night without a vehicle or doing business with the station and approaching random persons driving in counts as suspicious.
There is more they can do then "nothing".