I personally think that the solution to this problem (which I also reported here), shouldn’t be to only be able to disable the auto-completion, but to disable this behavior of being able to select a suggested value by only hovering the cursor over it (without explicitly clicking on it). To select the suggested value you should click it or hit TAB or one of the arrow keys (up or down, depending of its position in the suggestion list). Otherwise, if you “accidentally” have your mouse cursor over one of the itens in the suggestion list and hit ENTER, you will select the suggested value without meaning to. This still drives me mad about Coda.
A similar problem also happens in the autocomplete for formulas (which I also documented in my previously mentioned post).