How about this: after not having made a move in a particular game for six
months your rating disappears from the displayed rating list. If you make
a move again (or finish a game?) your old rating comes back.
I'd make it more frequent than that - every two or three months?
Also, I'd suggest that players who reject a game and who do not
currently have at least one of that game running should immediately be removed.
As Lyman suggests, removed names should probably appear in a separate
"Inactive" list appended to the bottom of the existing rankings lists
just to avoid confusion, but need not be included in the overall
"Established + Provisional" list.