Refusal of registration applies only to initial applications; cancellation requires notice and an opportunity to be heard. Refusal of registration is limited to initial registration applications where ex parte best judgement assessment grounds exist; where registration already exists or continues by declaration, the ITO must invoke cancellation procedures, giving not less than fourteen days' notice and a reasonable opportunity to be heard, and must record a separate positive order of refusal or cancellation rather than merely treating the firm as unregistered in an assessment order.