ماذا يفعل صاحب العمل في حالة ترك الموظفين وظائفهم للعمل لدى شركات أخرى؟

Unfortunately, they are unlimited contracts.

Can we ask them to sign a non-disclosure agreement before they leave because they have all information about our business and clients' data?

Of course, it is your right to ask them so, also it is their right not to sign this agreement.

However, there will always be a better solution, including the non-disclosure terms into your F&F "final and full" settlement or termination letter or resignation confirmation email and letting them sign it or acknowledge it.

In case you let them acknowledge, kindly include:

By receiving this email, you hereby acknowledge and confirm the full understanding of our non-disclosure terms and condition, in case we didn't receive any comments from your end within 5 days regarding the included terms, that will be considered as a full acknowledge from your side, also you hereby undertake the full sole responsibility that our company has the full right to lawsuit you in case of any breach to our NDA terms & conditions.