SRM new ui, disable the mark it as the assigned supplier button

Hi

We are using SRM 70 in classic mode.  For text item (non catalog) we do not want the requester to mark a sc item with mark it as the assigned supplier. Once its assigned the a po is generated once approved. So we would like to desable this button. We would preffer that the buyer does the sourcing accordingly. Is there security or field edit that can achieve this for us?

thanks