Grab this nifty script by SVDL which will allow you to turn off texture filtering for everything and just turn it back on for transmapped hair.  (Poser 7 texture filtering utility)

http://www.renderosity.com/mod/freestuff/index.php?user_id=99327

Also, I don't recall if P7 gives you an option for the post filter size and type.   If so, set it to 3 and to 'sinc' not the default 'box'.   Here's a screencap of PP2010 settings but if it's in P7 it will be in the same location probably lower right.

http://www.renderosity.com/mod/forumpro/media/folder_10/file_467191.jpg

I find a combination of these two give crisper renders.