Clean-out the bolt heads, strike bolt heads with hammer fairly hard (not so hard as to damage bracket bearing but OK to deform bolt heads somewhat), insert quality, unworn bit socket fully into bolt head with hammer if necessary, use quick snap of wrench to loosen (rather than slowly increasing torque). Unless the clutch is removed and you have a suitable holding fixture at this point you should probably have an assistant keep the fan from turning you you can use both hands on the wrench. Replace the bolts with new bolts if the heads are distorted/worn. Mark