When you don't specify UserDictionaryFile it defaults to ~/user-dictionary.txt, so that's why the button still shows. You should set UserDictionaryFile in both cases, but you can hide the Add button through customization:
https://keyoti.com/produ...idSpellWeb%20Layout.htm
I don't know if you're using the jQuery dialog or not. If you are, then it should be straightforward to hide the add button using Javascript. With the jQuery dialog the dialog code is actually on the page doing the spell checking - whereas with the ASPX page mode, the dialog code is in a separate window.
If you are not using the jQuery dialog (there is an ASPX page with the popup that loads in a different window), you'll need to follow the first section of that link above. The question will be though, how do you tell the popup that you don't want the Add button visible. You need some form of communication between the main page and the popup. Possibly adding your own GET parameter to the popup URL might work
rapidSpell.dialog_popupURL='path_to_file.aspx?showAdd=false';
and then in your popup ASPX you can read 'showAdd' in Javascript and hide the Add button with CSS.
Otherwise, you could use a cookie to communicate with the popup, set the cookie in JS just before launching the spell checker, and the ASPX page's JS read the cookie and hide the Add button.
That's a lot to take in so let me know if I can help further,
Jim
-your feedback is helpful to other users, thank you!