trace the circuit backwards to the point at which it branches to the fan and switch.
if the circuit doesn't continue on beyond the fan and switch, then undo the hot and neutral wires that branch to the fan and switch and leave them capped in the box.
if there's more stuff past the fan and switch, then just take the switch out of the circuit (cap the hot switch feed in the switch outlet box).
and always turn off the breaker before you start.
note - i'm not an electrician.