Can the pilot of a commercial airline refuse to fly if they deem the conditions to be unsafe?

Absolutely!

The PIC (Pilot in Command) is the ultimate authority of the flight. S/he can delay the flight for any reason (such as weather), refuse a plane they are not comfortable with (as in questionable mechanical issue), etc.

SAFETY is the very essence of the Pilot’s job!

Below: Me (R) and my 777 crew, preparing for our DEL flight. We’re happy that the plane is safe and ready! ;o)

Source: Quora