I would 301 redirect the old URLs. You want to retain the traffic for at least as long as other sites are linking to the wrong URL and certainly while the old URL is listed in the SERPs. Second best is to return 404, or meta noindex. Worst way is using a robots.txt disallow.
It will take at least a few weeks, often a few months for the SERPs to be cleaned up. It may take 6 months for the URLs to drop out of the WMT reports. That is NOT a problem.