I am saddened I had to go to these troubles. The matter should have been easily resolved had shotachan staff honoured my initial delete request. Had they acted professional, they might even have convinced another artist to join them.
So thank you all for you help!
However, for the fellow artists who told me it is pointless to complain about shotachan and those who said they only share their work there because they got bullied into it and that that is the only way to get at least some control over its distribution there:
You are not as powerless as you think. You have an audience. You have colleagues in the same situation. Ask for their help.
Shotachan staff are alone and they know it. They are afraid of the bad publicity their own actions will give them if made public.