You can restrict the Work selection that is visible for your employees when they are entering time.
On the Work settings, tick Job Limited Selection :
This Work will only show in Time Entry if the employee has a Job assigned to them for this work.
Normally only the jobs associated with the selected work are shown in the Time Entry screen, however, if the work has the box Applies to all Clients checked and the Job Limited Selection box is checked then after selecting the work in Time Entry all jobs assigned to the employee will be shown.
This can be useful in the situation where you have work such as Leave that isn't associated to a single client but you may want to invoice a particular client for leave taken while the employee is assigned a job for that client.