Yes this happens all the time and has done for many years now.
The FOS made a ruling a few years back, that Insurers are able to charge a time on risk, plus a maximum of £50 as a cancellation fee. So if you believe the cancellation amounts are wrong, then make a complaint. Insurers have up to 8 weeks to resolve, after which you can make the complaint to the FOS. As the FOS charge the Insurers a £500 case fee and the consumer nothing, many Insurers will choose to make an economic decision to waive the cancellation fee as a gesture of goodwill and provide a breakdown to justify the time on risk amount.
You can name and shame the company concerned, as that is not against the forum rules. You just can't slag them off using swear words or name company staff that you have dealt with.
There is also a review page on the Moneysupermarket Car Insurance page you can use.