Spam does not deserve a polite response, and responding to spam is, in fact, detrimental to the rest of the Internet and rude to the rest of us using it.
So you are being rude when you do respond to spam - and including LinkedIn spam.
Now you're just making up new meanings for these words. While I agree that you shouldn't respond to spam, there's nothing "rude" about responding to spam, and nothing "polite" about ignoring a message.
Spam does not deserve a polite response, and responding to spam is, in fact, detrimental to the rest of the Internet and rude to the rest of us using it.
So you are being rude when you do respond to spam - and including LinkedIn spam.