I don't think it is a good idea to automatic control Ignore button effects. Even with what LoyceV proposed (10 merits earned to be automatically un-ignore by the system), it won't make sense too. What happen with cases that abuse merit system? 10 merits won't be hard to abuse if they are abusers. So, you will have to read their shitposts again, and click on the Ignore button again. That's a boring cycle.
If you know  a guy, whom ignored by you for months ago, then you are curious to see that he or she actually changed posting habit or not. Simply click on Unignore to check. If you satisfy with changes, keep it unignore, whilst if you don't satisfy with minor changes or without change in posting habit, click on Ignore button again.  
Ignore > Unignore (to check potential changes of posting habit) > Maintaining Unignore / Reverting back to Ignore.