Almost one third of the working population in the UK has a second job. As an employer you cannot order your workers not to have a second job as it is classified as a restraint of trade. However you can insert a clause preventing your employee from doing anything that would represent a conflict of interest to your business or bring it into disrepute.
It is important to check contracts carefully to ensure that as an employer, you are adhering to working time regulations
and that the second job of your employee does not impact on their performance in their main role.