You can disable an Azure Function in a Function App using Azure portal.
If you open the Function App in the portal, you can find the
Functions blade and clicking on it, you have the list of your functions and can disable or enable each of them (using the ellipsis menù on the right).
You can also control the enabling or disabling of a function by changing the app settings of the function app.
In particular, you can add the following configuration key in the app settings:
<FunctionName> is the name of the function and the value is
1) to disable the function or
0) to enable it.