There's a strict policy regarding SPAM online, and companies can suffer lots from it. And I never saw a company that sends a newsletter that ever told me "look fella, you have to remember your pin code that you chose when you first signed up for this newsletter. If not, then you better have an account on THE portal so you can sign yourself out". No, they offer you the quickest possible way to do that.

you see, the big difference here is, that we don’t send the SMS. It’s all done on the device in an app you installed and configured.

However, you have two options now:

  1. if you still remember your avast! PIN, use this SMS command to deactivate Anti-Theft:
    the phone number you’ll send it to, is the phone number which sends you the (unwanted) SMS.

SET PROTECTION
Sample: 2222 SET PROTECTION 0

Will enable or disable Anti-Theft protection (0 = turn off, 1 = turn on)
Caution: By using this command you will
completely disable the avast! Anti-Theft system
thus rendering the Anti-Theft protection useless.

  1. If you don’t have the avast! PIN, you still have the phone number of the new owner.
    Call him and ask him to uninstall Anti-Theft.

You can send him this “how-to” for an easier uninstallation:

to uninstall our app please proceed as follows:

  • open avast! Mobile Security, open the settings and select the “avast! Anti-Theft removal wizard”. When it’s done you can uninstall avast! Mobile Security like any other app.
  • if you no longer have avast! Mobile Security installed and just want to uninstall Anti-Theft you can either reinstall avast! Mobile Security to use the wizard or proceed in these steps:

open your phone’s app manager and uninstall the app with the custom name you chose on installing avast! Anti Theft. In many cases the name is “Update Agent”.
If you can’t find Anti-Theft like this, please go through all installed apps. Look for apps you don’t know what they are doing. If you come across an app you don’t know, select it in the app manager.

You’ll see some options along with a list of permissions the app requests in the android system. If the list matches this list:
https://support3.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1252&nav=0
you found Anti-Theft. Just uninstall it.
Of course you can just uninstall all apps you don’t know, as it’s most likely that you don’t need them if you don’t know them. You can’t ruin anything here, as all system related apps are uninstall protected.

If you find the app and the uninstall fails, please remove the app from the device administrators list (you can find it in your phone’s settings).
If the uninstall fails please remove the app from the device administrators list (you can find it in your phone’s settings).

Regards,
Werner