One way to do so would be to make it so wireless companies can lose access to spectrum as a consequence of customer data breaches. Let someone else who can keep customer data secure have it instead.
I think financial penalties are still the best bet if they are large enough to really hit profitability but not large enough to kill the company.
Look - too big to fail means we let too many companies merge. This isn't a healthy situation that losing T-Mobile means having no competition left. We should probably unwind some mergers first.