Welcome to Flyertalk and welcome to the BA forum.
Unfortunately this is something that does get reported here every so often, there is clearly quite an industry out there stealing Avios. From past reports, and to set expectations, it may take 2 - 3 weeks before BA reply to you on this. When they do, it may have very little details about what has happened or what they are doing, merely advice on how to re-enable your account.