That's the problem with season ticketing in seated areas. Corporates were buying up season cards in doubles and blocks from day one and just renew every season. They give them to friends/clients etc. Sometimes they go, most games they don't, but they don't put them on the ticket exchange. You know exactly which games they will be used...
The sooner the club deals with unrealistically low usage and blocks the practice, the better.
One guaranteed solution is to change season cards to 'priority booking' cards, requiring holders to buy tickets for their existing seats on a match by match basis at a reduced rate. If they don't buy their ticket within seven days of the game, the seat goes on open sale at full rate.
Involves a bit more effort on the part of regular attendees, but not really much more than the old 'pay on the gate' system.
Low usage card holders (say less than 10 games per season) would simply lose the right to a specific seat, or be reallocated to row ZZ of SS3....