This is a legitimate concern.
1. If you're going to be giving them a steady flow of work, it would be unwise for them to poach your clients (as they would lose future work) - make this clear to them.
2. Have them sign a short confidentiality & non-compete agreement (I can help you draft one).
3. Get references before you start using any particular company/freelance - it's fairly easy with today's freelance platforms and/or using LinkedIn.
4. The obvious: give them the minimum information possible (at first, until you learn to trust them).
I've successfully helped over 300 entrepreneurs and businesses and I'd be happy to help you.
Good luck