To be specific:
Go to Configuration> Advanced Services> All Profiles. Expand AP> Regulatory Domain. Type the Name of your New Regulatory Domain Profile and Click on Add. Uncheck all of the channels you don't want, and leave only the single channel that you do. In the example below, I only have channel 36 checked:
Click on Apply.
There are two ways to Apply this to an AP: (1) Either at the AP-Group Level or at the (2) AP-Specific Level:
(1) Go to Configuration> Wireless> AP Configuration. Click on the ap-group with the mesh devices. Expand AP, click on regulatory domain, and change the dropdown to the right with the regulatory domain profile you just created and click on apply. All APs in that ap-group (mesh and non-mesh) will only use channel 36 (or whatever you left checked) immediately on the 5ghz band if they have ARM enabled (they do, by default). You might have to create a separate ap-group for mesh and non-mesh devices, as a result.
(2) Go to Configuration> Wireless> AP Configuration. Click on the AP Specific Tab. If you don't see the name of the mesh AP you want to configure (start with the mesh portal first) you will have to click on add. When you click on add, find the AP you want to configure and click on add. Click on the name of that AP and expand AP, click on regulatory domain and change the dropdown to the right with the regulatory domain profile you created before and click on apply. That AP will be constrained to that channel or those channels immediately.
You can also use this method to quickly create a static channel plan by creating regulatory domain profiles with individual channels and using the ap-specific method to apply them. I hope this helps.