Simplest solution is to go for a strict force-a-side defense. There's at least some chance the defender can watch both the disc at the current handler and keep track of Bob's actions in the corner of the eye. If the marker on the handler enforces one closed side, the defender can put himself clearly on the open side and watch both. Of course, there's no ideal world, and no side is always 100% closed, and there's overheads and hammers and such. But it does work rather well, especially if the team is trained well in forcing a side.
A system my team once played is rather hard to do: We called it the "buddy system": Every player on the field has a buddy on the outside. Before the game the team agrees on active players either watching the handlers or watching the respective marked offense player. Let's say for example the team agrees to always watch the handler. Then each buddy on the sideline watches the marked offense player and shouts his direction of movement to his active player on the field. There are several points to this:
- The active players on the field must trust their buddies and must keep the eyes on the disc to not miss the throw.
- They must trust that their buddies shout correct instructions
- They must listen to their buddies.
- In turn, the buddies must shout correct directions ("left", "right", "fore", "back" or similar) and they must modulate their voice to the urgency/speed/speed change of the marked player
- The buddy pairs of active and sideline player must know the voices of each other well. Obviously there's no point if there's no 100% understanding.
The result is a cacophony of voices at the match which can be quite intimidating. Some teams may consider this foul play because they mistake this system for an aggressiveness. The point really is that there's no possibility for any kind of trick play such as the "iso bob" mentioned in the question.
Imho iso plays will always be popular, but they cannot be used to win a game. It's one of those tricks an experienced, competitive team should have up their sleeves and pull out of the hat once in a while.
Disclaimer: I'm a geezer now, too ;)