You can't do it with official Virtuemart. That select box is a "Sort by" drop down, and there must be at least 1 sort by rule (in your case, it is SORT BY: PRODUCT NAME). But luckily you are using one of our templates ;) and you can hide it.
1- Navigate to Joomla Backend >> Plugin Manager
2- Open this plugin: "Flexible VM2 Template"
3- Find this parameter: Show Sort By and set it to NO
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The sort by select box will be gone in the category browse page.