Are you a commodity business or a value-add business? Because I'll give you a different answer.
When choosing a development shop, I advise my clients to figure out where in the organization technical leadership resides.
If you are a value-add business whose processes and procedures train developers how to deliver high-value, then the technical interview should be you and your technical leadership team, and should go into what you train, who you hire and why, etc.
If you are a simply providing access to developers as a commodity, then you need to expect people will want to do a technical interview of the developers to learn how they will approach their project.
I also advise clients to interview anyone they will work with directly, just to make sure they are good problem solvers and communicate well.
Clients are getting much smarter to the fact that there can be a huge difference in productivity and quality between seemingly identical developers, and a few carefully chosen interview questions can expose that difference. If you don't want to allow interviews, you need to figure out how can you proactively show productivity and quality standards to your prospects and clients.