Hi @Tom Hamilton,
Welcome to Microsoft Q&A!
The autocompletion feature is hard to disable completely. To disable it, I suppose the best option is to toggle IntelliSense completion mode by pressing shortcut Ctrl+Alt+Space. Or you can change the default IntelliSense completion mode to Tab-only: Tools > Options > Text Editor > Advanced > Default IntelliSense completion mode: > select Tab-only in the dropdown.
If you use the C# language, maybe you can try the third-party extension to disable it.
Sincerely,
Anna
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