The drop-down list of possible items to add as depict tags should not include categories.
Those aren't categories, those are Wikidata items containing the concept of Commons categories. :-(
Right, sorry. Nevertheless, you should not be able to select them, and they should not appear in the drop down menu.
Excluding them is going to be expensive and slow (pipe the [[https://www.wikidata.org/w/api.php?action=wbsearchentities&search=Cat&language=en&props=|wbsearchentities]] into [[https://www.wikidata.org/w/api.php?action=wbgetentities&ids=Q4167836%7CQ146%7CQ14819232%7CQ32566%7CQ5159797%7CQ14849060%7CQ912640&props=claims|wbgetentities]] looking for P31 -> Q4167836 and filtering them out.
Note however that I don't get the category results when searching the real Wikidata – possibly there's a hack on the search config to not return those?