As far as I'm aware a clean reinstall of avast 2015.10.2.2215 the current regular release (see instruction and links below), should wipe out the history of your having been a beta build user.
EDIT:
If you stick with what you have now - the next round of beta testing or another beta build in this beta session, would result in your being offered the new beta as a program update.
You don't say what avast version that you are using, the instruction and link below is for the free version.
Download the latest version of avast, 10.2.2215
http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, avastclear.exe
http://files.avast.com/iavs9x/avastclear.exe and save it to your HDD.
- 1. Now uninstall avast! - using add remove programs/Programs & Features, if you can't do that (as appears to be the case) start from the next step, reboot.
- 2. run the avast! Uninstall Utility, it will request it be run from safe mode (and will handle the boot into safe mode), once complete reboot into normal mode.
- 3. install the latest version, reboot.
Note: save the copy of avastclear.exe so it can be reused as some users are reporting that after running it, it is self-deleting.