If I understand you properly, you got the sign out of the machine but you can't get the plexi-glass out of the sign frame?
Right. There where two L type brackets the back of the frame held in place with clutch head screws. After removing those i found a rivet i had to drill out as you suggested to take it apart. The nice part seems to be that the rivet wasn't really required, as those brackets hold the sign together just fine even after the rivet was removed.
Thanks for the tip.