Once buy for customers.

Hi,
Thanks for getting in touch with us!
Sorry, I couldn’t fully understand your query. If I’m correct, you want to sell your products (virtual items) only once. You can achieve this by configuring it’s inventory. Go to Inventory tab and from there enable Manage stock?. Set Stock quantity as 1 and Allow backorders? to ‘Do not allow’. Ref: https://imgur.com/0Gl8WuW
Looking forward to helping you.
Thank You!

Well, hiding inventory tab from vendors can be easily done from WCFM admin dashboard >> Capability page see here- https://imgur.com/lUQT1KG. But setting all the products inventory values via code requires customization. You can use the following snippet (will not work on variable product type) to get the idea-

add_filter('wcfm_product_data_factory', function($data) {
    $data['manage_stock'] = true;
    $data['backorders'] = 'no';
    $data['stock_quantity'] = 1;
    return $data;
});

Add this code to your child theme’s functions.php
In case you do not have child theme then add code using this plugin – https://wordpress.org/plugins/code-snippets/
Let me know how this goes.
Thanks!