Hi [Recruiter Name],
I hope this email finds you well. First, I would like to say that I am grateful for the opportunity to have been considered for the [Role Title] internship with [Company Name]. Furthermore, I really enjoyed speaking with you and the rest of the team. However, I have recently accepted an offer elsewhere for the upcoming summer and am unable to move forward with the internship process with your organization.
Nonetheless, I think [Company Name] is doing amazing work and I would love to be kept on your radar should any future opportunities become available. Once again, thank you for the opportunity!
Best,
[Your Name]