Assuming it's 4 octocore processors, take out 2 of them.

Edit: Sorry, that's a bit abrupt but it's the quickest way I can think of 🙂

Alternatively create a VM on the server, assign max 16 cores to it & install SQL on that.

I'm not up on SQL 2014 licensing but in earlier version MS didn't allow you to use processor affinity to control licensing so can't think of other ways.