Bathroom Exhaust Fan Keeps Running
A light switch is a mechanical device with moving parts. They get used many thousand times over their lifespan. The switch could be bad. It is possible that in your case the failure is that the switch is staying closed, i.e., ON,  when it is flipped OFF (the position in which it is supposed to be open).

Turn off the breaker to that particular switch. This should turn off the fan. (If the fan continues running, then either you have turned off the wrong breaker, or the workers wrongly connected the hot from another circuit to the fan so that it is being fed by two breakers. The contractor must correct this.)

Remove the two wires on the switch that controls the fan and move or bend them so the bare ends are out of the way. Neither of two ends should be hot (breaker is off) but if you have a non-contact tester make sure neither is hot.

With a multimeter in resistance mode touch the leads to the two screws on the switch from which you removed the wires. This is a check for "continuity", meaning if the switch is ON then you should read close to zero resistance (say micro ohms). If the switch is OFF, you should read very, very high resistance (mega ohms), meaning the switch is "open".

If you have low resistance when the switch is OFF, then the circuit is not being broken when you flip the switch OFF. Replacing the switch should fix the problem.

Replacing a switch is a relatively cheap way to begin troubleshooting your issue.

EDIT But consider the possibility that the fan has a moisture sensing switch in parallel with the manual wall switch. If you have this and either the sensor is faulty or a source of water is wetting the sensor, then this could keep the fan running even when the manual wall switch was OFF.

Moisture sensing fans do exist but one would have to know if they were supposed to be connected in parallel with a wall switch or in series with a wall switch, and could be both with two different wall switches but would probably be only in parallel.