Is it possible to disable the default URL assigned to the azure web app?

An App Service might be running in an App Service Environment though, and their default hostnames are different to .azurewebsites.net (see https://learn.microsoft.com/en-us/azure/app-service/environment/how-to-custom-domain-suffix), I believe you can even select your own domain suffix that replaces the one for the App Service Environment.

Additionally, App Services in Clouds other than the Global Cloud (GCC High, DoD, 21Vianet) use other domains than azurewebsites.net.

Thus, it is difficult to impossible to disable it for an existing Azure App Service, but if you are dealing with App Services in general, you should not assume that the domain always ends with .azurewebsites.net.