I'm not really here
Join Date: Aug 2006
Express thanks for their work and support.
Explain that they're services are no longer required.
Explain the reasons for that decision (which I presume is just, and can be evidenced to be so).
Hold your line.
If it's a charity I'd be tempted to check your constitutional documents just to make sure there is nothing weird in there. Ultimately, though, you need to be acting in the best interests of the organisation.