As an employer who is looking to take on applicants on to a training programme with a view to developing them and promoting them in the company long term. Do I have to take on applicants who are on a dependents visa

For dependent visa holders, legal status depends on the principle visa holder, and can be terminated if the principle visa is expired, or they are divorced. If the company needs to invest such as offering trainings and the jobs are long term, I would not recommend recruiting anyone on dependent visa.

Be more specific in your vacancies ads that eligible applicants must be permanent residents only.