Second this approach. If someone interrupts, just keep talking. Do not back down before the interrupter stops. Think of it as a conversational filibuster :-)
If both parties keep talking it will get awkward for everyone in the conversation. This is what you want, as it will lead to immediate understanding that an interruption happened and that it's not OK. You can calmly discuss afterwards that you were interrupted, and after a few occurrences even those with only a modicum of social awareness will get the message and change behaviour. And peer pressure can then help those stragglers.
If both parties keep talking it will get awkward for everyone in the conversation. This is what you want, as it will lead to immediate understanding that an interruption happened and that it's not OK. You can calmly discuss afterwards that you were interrupted, and after a few occurrences even those with only a modicum of social awareness will get the message and change behaviour. And peer pressure can then help those stragglers.