If they're concerned about exposure, they could simply set the margin trading leverage below 1 (e.g. offering 0.1x leverage so that customers have to post $10 in margin to buy $1 in MicroStrategy shares).
> Are you saying someone would need to post more than the value of the share?
Yes, as an alternative to not allowing the trade.
If the bank considers something highly risky, it makes sense to protect themselves and the customer by making sure they've got some money locked up outside of the trade (a bankrupt customer is no longer a customer, after all). It would basically cap the percentage of net worth that they could put at risk, where they'd need to have $10 set aside for every $1 in the risky position.
There’s no need for a bank to nanny their customers. If you don’t want to book the trade, don’t book it. Going beyond that in some tortured logic to ensure they tie up extra money (possibly incurring unjustified margin calls on other positions) has almost exclusively downsides for HSBC.