Is there any rule that would prevent former champions and serious competitive skaters from entering Adult Skating contests as ringers and winning all the prizes?
Theoretically, they could, but why would they want to? The only reason I could see would be if they "hung" up their skates for a awhile and then wanted to relive their childhood years again. Most skaters who have competed at a high level usually move on to tours or teach.
Well, Michelle Kwan might want to add to her medal count so no one would ever be able to catch her as the "most decorated U.S. skater of all time."
Former elite skaters who are adults are allowed to compete in adult events at the appropriate levels.
At US Adult Nationals, for instance, there are adult bronze, silver, and gold levels for skaters who either started as adults or did not reach a high test level as kids.
There are also "masters" levels for skaters who have passed at least the intermediate tests. In the open events there are masters novice, masters junior, and masters senior events, divided by age groups if there are enough entrants to warrant it.
There are also championship masters events that mix all the masters levels together. Skaters compete at adult sectionals to qualify for these championship events.
Former senior-level elite competitors could either qualify through sectionals for the championship event or enter the masters senior event, in their discipline, or both.
The international adult event in Germany seems to break down the higher levels into "masters" and "elite":
http://www.isufs.org/results/adult2008/
Here are the results from 2008 US Adult Nationals:
http://www.icenetwork.com/events/detail.jsp?id=34667
Of the names I recognize, the championship masters men's event includes Larry Holliday, who was a national-level skater at the senior level in the late 1980s/early 90s, and Edward van Campen, who competed internationally for the Netherlands in the early 1980s.
In adult championship pairs, Craig Joeright, an international US pairs competitor just a few years ago, competed with a partner who took up pair skating as an adult.
There is also an event called "Centennial Dance" in which the combined ages of the two partners must be at least 100. The roster includes Sally and Stanley Urban, who competed internationally for the US in the 1960s.
Maybe Tonya Harding would want to compete to get the medals she missed while competitive.