After subscribing to any mtn's data plan, your sim card automatically goes into an auto renewal loop. This means that once your data subscription period expires, your sim card automatically renews your subscription without your permission. This also deducts money from your available balance without your permission.
To prevent this, simple send NO plus the subscription code to 131.
For example if you subscribed to mtn's daily plan for 100Mb, you have to send NO104 to 131 after subscribing.
Check the chart below for the subscription codes.

30MB @ 100NGN = 104,
100MB @ 200NGN = 113,
750MB @ 500NGN = 103,
1.5GB @ 1000NGN = 106,
3.5GB @ 2000NGN = 110,
10GB @ 5000NGN = 116,
22GB @ 10,000NGN = 117

culled from http://enigeria.com.ng/must-subscribing-mtns-data-plan/