I think it's possible to implement, but it's very tricky. There is no system event triggered on plugin save. There is such events for content items - onContentBeforeSave, onContentAfterSave, but there is not for plugin options.
The tricky idea is to check with the URL if current action is your plugin save action. You'll not be able to interrupt it, but you'll be able to revert changes later. I'm able to create such methods, but have no ready solution.
So here is some code you can use as a reference.
Here is a fuctnion to check if the current action is plugin save action (works together with the construct method in this class, which sets some internal variables like
Next here I run the method
which sets a boolean flag.
And here if the plugin has been just saved a run a function
That function loads plugin options
So at that point you may get just saved options, check it with your methods and if something is wrong null the parameter and resave it to extensions table. And call a regular Joomla message saying something bad. Or even redirect to the plugin edit page.
Maybe you should better use some custom field with ajax to check the field on the fly before the plugin save. It would be better as for me.