How would I limit speed of dc motor being used as a generator?
A few ideas: 


Rotate the motor so the blades are out of the wind
Change the pitch of the blades by rotating them at the base, so they are no longer effective, if they can have that capability. Also called 'feathering'. 
Use a mechanical brake (you could use a mechanical clutch but that would allow the blades to turn faster, which could cause problems). 
Apply additional load to the motor, dumping heat into a dummy load if necessary