What was your original visa? Did you arrive on a tourist and then change to A Non B in Thailand using their company paperwork? Or did you have the Non B issued abroad using their company registration paperwork to obtain a Non B? If you used their company registration etc to get the Non B and then you decide not to stay. Normally the visa is issued under the company based on employment and then obtaining a work permit. If you leave then the process is stopped but the Visa would be valid as long as it was in the first 3 months. If its an extension then it would be void if you no longer work for that company.