Panama is probably the one country that could hold it out - retaliation on the Canal would escalate matters to a point where rich countries have to choose between a dangerous military occupation or sitting at the table with the local government.

But yes, isolation of countries like Bermuda and Cayman Islands could achieve a lot very quickly. To be brutally honest, the UK government could shut down most of them tomorrow, if they wanted to; but they have a few incentives to do only just enough to appear like they want to, without actually doing so (going from nefarious "their own moneyed citizens want to keep money flowing" to relatively innocent "they risk losing whatever little formal power they still have on former colonies that they can't directly occupy anymore").

I wonder how essential the Panama canal actually is. If the Panamanian government decided to play rough and use it as a threat, there are several alternatives for global shipping. Some could use the Suez canal instead with only a minor increase in shipping time. Others could use rail, and the worst affected could still go through the Magellan straight.