There is a simple workaround to select a distinct value in your example. It is correct, that "&select=Document\LB01,Sales" will select all values which contain the word "Sales" ... so something similar to *Sales*.
Having said that, just create another Listbox (e.g. LB02) and put an expression instead of the field. If your values were in field "category", the formula would look like:
The undesired behavoir of "select where the word is contained" is no longer applicable, since there is no space in the options of that listbox and you may select explicitly one value with the querystring