It's a well-known problem that Android offers no easy way to completely remove the primary Google account, at least sometimes.
Well, there's one not really easy for people not involved with computer stuff, but at least it works.
Once you have adb shell working, you need to log into your Android device and type sqlite3 /data/system/accounts.db.
That's the place where accounts are actually stored. In that database, there are tables named accounts, grants
and authtokens. Those you need to clean up:
SQLite version 3.5.9 Enter ".help" for instructions Enter SQL statements terminated with a ";" sqlite> select _id,name from accounts; 1|user@gmail.com sqlite> delete from authtokens where accounts_id=1; sqlite> delete from grants where accounts_id=1; sqlite> delete from accounts where _id=1; sqlite> select _id,name from accounts;
After that it may be a good idea to restart the device, but I'm not sure it's really needed.