Former World Heavyweight Boxing Champion Roy Jones Jr. asked Russian president Putin for Russian citizenship when the two met in Crimea (a territory recently stolen by Russia from Ukraine). President Putin recently granted that request.
Section 349 of the INA (8 U.S.C. 1481), as amended, states that U.S. nationals are subject to loss of nationality if they obtain naturalization in a foreign state upon one's own application after the age of 18 (Sec. 349 (a) (1) INA).
Since Roy Jones Jr. is eager to be a Russian, and by his recent actions helps to legitimize Russian aggression in Crimea, there is no reason to let him maintain his U.S. citizenship.