Perhaps a heat gun, rented from a local home improvement type store. Its a super hot and will remove paint, but I implore great caution. There may be a clear coat over the non-decaled, satin finish areas of the bike, so test an inconspicuous area. The thing is also hot enough to melt grease out of bearings, burn rubber, not to mention flesh, and melt plastic, maybe even some plastic you can't see buried in hubs or BBs. It would be safest to burn the decals off the frame after it is stripped bare.
Furniture finish remover might work also, with the same cautions.
Both of these options seem to be a royal pain. How annoying are the decals..............********************??