Disable dropdown multiple selections
When you turn on the search box with taxonomy/attributes or customer field dropdown menu, by default you can select multiple values for filtering. Take a look at the below image.
If you want to turn off multiple selection values, use our 'wpto_is_multiple_selectable' filter hook.
Add those codes in your theme/child theme's functions.php file or use 'Code Snippets' plugin to add those codes.