Is a female employer entitled to prevent her female domestic worker from using a mobile phone permanently, and to allow her to use it once a week, after discovering she has multiple relationships with foreign men?

The employer does not have the right to prevent the maid from owning her personal phone. The fundamental principle is to have a good opinion of her, for Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." If she commits a wrongdoing, she should be advised with wisdom. If her corruption is feared, then she should be dismissed; because the presence of maids can be a cause of many evils.