Bestmark is one of the only companies I ever deactivated from. It was many years ago and during a phone call where they informed me that they would not reimburse me for a correctly performed shop since the time on the receipt did not match my report (despite the fact that the shop was performed one day after the daylight savings change and my timings were correct, but exactly one hour off from the receipt).
It was early in mystery shopping career and did not know back then to even look for such discrepancies. In retrospect, I consider myself equally at fault since I did not bring it to their attention before submitting the shop. However, they were inflexible in their decision to reject my shop and after I appealed as far up the chain as I could go, I requested to be deactivated out of anger and frustration.
Years later I missed having access to some of the dining shops they offered and emailed them, requesting reactivation. Like Mary, I never got a response. I reapplied with a different email address and they accepted me. All of the other info I supplied was the same, so I didn't see anything unethical about it. I think the accounts are tied into your email address and nothing else.
I guess the good thing about a company that doesn't really care about shoppers as individuals is that they don't really take the time to comb their database for duplicates.