Barred

I interpret this as meaning behind bars. On the other hand, Bradley (48), Halton (147), Terry (286), and Walmisley (299) propose that the term refers to a rule of billiards. Terry puts it thus: "… after the red ball has been 'potted' (= pocketed) a number of times, that stroke is barred. The balls, on being taken from the pockets, are placed on spots on the table: hence the term." I invite them to rethink the context: "He's made to dwell in a dungeon cell on a spot that's always barred. And there he plays extravagant matches … " In Rees's view, Gilbert may have intended the second meaning to be taken as a pun (251). Karr (171) and Kesilman (174) concur. Perhaps we can all accept that compromise, which gives double value to the term.

Context: 
On a spot that's always barred
Opera: 
Act: 
Act II