It depends on what it is that makes the individual unsuitable and whether or not the employee is unable to meet a condition of employment, such as 2 satisfactory references. Take great care and seek a legal opinion if you think there is any risk of a claim for discrimination. Make your offer of employment subject to certain conditions being met and continuing to be met throughout employment, according to the nature of you business. Conditions could be 2 satisfactory reference, CRB check, proof of driving licence, etc.