Good question. I think the most logical solution is that you don't get to place any new piece, but you have to move your counter as usual to end your turn. I don't think the player should be penalized or anything... There are a few other cases where an instruction cannot be executed, although the ones I can think of right now all mean you lose the game. For example: sitting on a lonely move, erase or jump (if there is only one other block and it's the one occupied by your opponent) piece. In fact this last case came up in our last game. By merging the only unoccupied block with my own, I left my girlfriend which had her counter on a lonely jump piece, with no place to jump (you can only jump to pieces from your own active block or to unoccupied blocks).
Of course, if your instruction counter is on a lonely write instruction, then you lose the game, because you cannot move your counter at the end of your turn.
BTW, you know you can use the capture pieces in future move instructions, right?
I'll add the answer to the talk page too, just in case.
Cheers, and thanks for trying the game out