In OpenERP, this is achievable by customising a price list, and then by assigning it to a partner, but with no category tag.

In my knowledge, actually"Tag for partner Category" is used as criteria for search in partner list.

I don't say that this is not possible, this can be done by creating a specific module if necessary, or by altering an existing one (which is not a good practice).

Try to read this article and this one

If you need more, you can also look at this link, to see if you can find what you need pricelist_by_category_qty and pricelist_by_partner