(take out the spaces in < style > and </ style >.)
but that will also hide any other RSS feeds you have running around your network, so to confine it to just your members pages, you could stick:
.module_feed {display: none !important;}
into the advanced box on your profile's theme editor page, or you can make it hidden by default by diving into your source code and editing every CSS theme file avaliable to your users by adding that code at the bottom..
if you have source code access, go to your root folder in any FTP thing, and then go to xn_resources/widgets/index/css/themes/
you can either edit all of the *skin name*-custom.css files or all of the *skin name*.css files, theres no need to do both.
if you do it to all of the *skin name*-custom.css files your members will be able to delete the section of code themselves and bring back the RSS box, but if you put it in the *skin name*.css file, your users will not be able to bring it back so easily...
i hope ive been of help, theres probably some PHP hack to do it too, but CSS works just as well ;)