It's impossible to disable the Partner Network feature - any time someone logs in via your website, the link to your website pops to the top of the list, so it's more visible to the people.
Partner Network ( http://gra4.com/networks ) just gives you another link to your website, that's it. So, it can't be more responsible for the spam than, say, Google.
Partner network points to your website, not to your profile, therefore it can't be the reason for the friends requests - potential friends probably found you via members list, activity stream, "Know them?" widget (implements "friend of friend" search), etc.,etc.
Any member of the GRA4 Network can make a friend request to any other member (no matter the website they are logged in from) - it's Social Network, so that's the idea =) If you don't want to be friends with someone - just reject the request. Even if you accept the request, you can "unfriend" any person at any time.