A now-broke businessman who once played a role in the Smurfs cartoon empire admitted Thursday he threatened to destroy his son-in-law’s stature in high finance as part of an $11 million shakedown plot.

Stuart R. Ross pleaded guilty to attempted grand larceny, acknowledging he demanded $5.5 million to end a strange campaign of threats to ruin son-in-law David S. Blitzer’s career as a senior managing director of The Blackstone Group LP, an asset management and financial services firm. Ross, 73, also admitted saying he’d give up any rights to see his grandchildren for another $5.5 million.