The International Cycling Union has threatened cyclists with one-month bans if they compete alongside suspended riders, even in unsanctioned races.
Hamilton tested positive in September 2004 for blood transfusions
The ICU has acted after American Tyler Hamilton was photographed racing in the Stazio Criterium Series in Colorado.
The Olympic gold medallist is serving a ban after failing a blood Test in 2004.
And a UCI spokesman said: "We can't stop banned riders taking part in unsanctioned races, but we can issue bans to riders who compete with them."
Hamilton, 34, tested positive in September 2004 for blood transfusions, a month after winning gold in the road time trial at the Olympic Games in Athens, and is suspended until early 2007.
His appeal against the ban was rejected by the Court of Arbitration for Sport in February this year, and UCI spokesman Enrico Carpani added: "If he wants to take part in unsanctioned races, he can, and we can't stop him.
"But all other riders and all other teams then cannot participate in that race. If they do, they can be suspended."
Hamilton dropped out of the race in Colorado after the UCI contacted USA Cycling.