Open the file

components/com_virtuemart/themes/default/templates/pages/shop.ask.tpl.php

(if "default" is the name of your theme.)

Open this in an editor, "select all" and delete it all.

This will remove the form, so it is impossible for the spam spider to fill it in. 

Now remove the link to the form.

Open

components/com_virtuemart/themes/default/templates/product_details/flypage.tpl.php

if you are using that flypage.

(the flypage is what VirtueMart calls the product detail page)

Now find line 43

$ask_seller

When you find it, you will see it is a full php tag, you need to remove it all. 

Save the page and upload it back to your site and you should find there is no more link.

Thats it, you will "never" get another spam again ;)