If you mean the portal box then disable it in the portal settings; if you mean the search portion of the header then you will need to manually make a change to the coding in templates/default/header.tpl. You will need to adjust the code to link to the search page versus using the dropdown.
I'm not at my computer right now to post code but if you can't get it figured out and no one posts it before I get back I'll post it when I can.
