There are few ways to stop auto deletion on your DF account. One is to buy credits or Gold Member with real money. Once you bought the member and/or credits with your money by using phone payment or credit card, the Admin will know that you payed money for playing DF and they will stop the auto deletion of your account.
Another way to stop Auto-deletion is to reach up to level 51.

Account Inactivity:

Level 1 - 10 accounts get deleted after 7 weeks of inactivity.
Level 11 - 20 accounts get deleted after 11 weeks of inactivity.
Level 21 - 30 accounts get deleted after 15 weeks of inactivity.
Level 31 - 40 accounts get deleted after 19 weeks of inactivity.
Level 41 - 50 accounts get deleted after 23 weeks of inactivity.
Level 51 - 220 accounts are safe from auto-deletion.