This is not the best of advice, but it is what I did the first time I came to Thailand. I had applied via email to some pretty shit agencies, the sort that I was convinced would hire anyone and everyone. I was offered a job (some silly amount like 25k), accepted graciously and explained how I would need a letter of invitation before I left. The sent it out, I got a multiple non-b (15 months, in theory) and then sent them a nice email to the effect that i was very sorry, but would no longer be able to work for their company.
I suppose they could have cancelled the visa, but I don't really see how that works. Every time I leave the country on a multiple non-b, then come back with a fresh 3 month stamp, my passport hardly gets looked at, and I am pretty sure that they don't type in my visa number to see whether it has been cancelled or not.