In geodirectory/geodirectory_template_actions.php around line 875 you find
add_action( 'geodir_add_listing_form', 'geodir_action_add_listing_form',10);
From your theme’s functions.php file you should remove that action.
remove_action( 'geodir_add_listing_form', 'geodir_action_add_listing_form',10);
Copy the function geodir_action_add_listing_form (it’s right below the add_action in geodirectory/geodirectory_template_actions.php) paste it in your theme’s functions.php file and rename it. Example my_geodir_action_add_listing_form.
Now modify the function to remove the text input field and use a select with your predefined tags instead.
Add a new action in your functions.php calling your new function.
add_action( 'geodir_add_listing_form', 'my_geodir_action_add_listing_form',10);
Let us know how you went.
Thanks