Looking through the books and Pottermore show no additional instances of anyone attempting to steal the stone.

As to whether it was guarded, Flamel himself is an extremely clever/gifted wizard shown by his ability to create the stone to begin with, so he would be more then apt to protect the stone from any ordinary threats, Voldemort being the exception as an extraordinary threat.

Looking through the books and Pottermore I've found no additional instances of anyone attempting to steal the stone.

As to whether it was guarded, Flamel himself is an extremely clever/gifted wizard shown by his ability to create the stone to begin with, so he would be more then apt to protect the stone from any ordinary threats, Voldemort being the exception as an extraordinary threat.