I have never attempted to do this, however I believe you may still have to Disable the Avast Self Defence module to do this.
I'm not sure what exactly was meant by "Avast Sef Defence Module" (only have options to disable "Core Shields" which seems to encompass File, Behaviour, Web, and Mail Shields, but the menu structure of Avast is
unbelievably complex) but this didn't work for me. Once you have logged on as a different user and deleted ("removed") the user in question, changing this option doesn't seem to affect files in the deleted user's profile. Not a big surprise there.
I uninstalled Avast and selected the option to remove all settings, then I was able to change security permissions in the affected user's folders and delete the entire user sub-folder from the C:\Users folder. Then reinstalled Avast again. PITA but then if you are using Avast you should be used to constant annoyances.