Hi and welcome to Advanced Installer forums.
To block the user from changing the installation location of your application, you can simply go in the "Dialogs" page and delete the "FolderDlg" dialog. However, the user may still be able to change that path by setting the "APPDIR" property from the command line.
In order to prevent this too, you can go in the "Custom Actions" page and add a "Set installer property" custom action after the "Wizard Dialogs Stage -> Searches" action group which sets the "APPDIR" property to the path that you want. Also, right-click on that custom action and select the "Sequence -> Install Execution" option to share it between the UI and the Install stages.
Let us know if this helped, otherwise give us more details about your scenario.