If you're a Gold or Platinum* account user, you can avoid unwanted charges by blocking access to calls, messages and other chargeable activity. All from the dashboard of your my.plan account.
CONTROLS > PREMIUM & ROAMING > PREMIUM CALLS [which opens access to all preventable calling options] > [select user] > BLOCKED
CONTROLS > PREMIUM & ROAMING > ADULT PREMIUM TEXTS [which opens access to all preventable messaging options] > [select user] > BLOCKED
*Essential account users require an upgrade