Some people have solved this issue by simply putting the bung through a dishwasher cycle, AeroPress themselves recommend taking the following steps:
1. Remove the seal from the end of the plunger.
2. Boil some soapy water in a sauce pan. (Just use hand dish soap) 3. Remove the pan from the stove.
4. Soak the seal in the hot soapy water for about 5 minutes.
5. Remove the seal from the hot water (use tongs or at least a fork so you don't burn your fingers)
6. Immediately aggressively wipe the hot, wet seal all over with a paper towel.
Repeat these 6 steps if needed. You can get the seal back to looking and feeling like new. When done, place the seal back on the end of the plunger.