Can old coins be banned by a soft fork?
Yes, since blacklisting a set of UTXOs would be done by adding more rules and thus restricting the set of allowed transactions, technically such a change could be introduced as a soft fork by the miners.

However, I would sincerely doubt that censoring Satoshi from spending her coins would find broad community support. I suspect that such an endeavour would quickly escalate the already existing tensions between community and miners.