As an agency, how do I know you’re not going to engage with our clients?

We address non-solicitation and privacy in our contract:

At all times during and after, The Consultant shall keep confidential any information about the Company’s clients, and not solicit or engage with their clients. The Consultant will delete any data and verify its deletion upon written request by the Company within 30 days. The Company will determine if and when their clients are emailed reports, and what email addresses will be sent to, and in what frequency.