I hate to sound like the big idiot here, but what exactly in the email you received indicates no-ip.com spammed? It looks to me like you just have some secret "admirer" who thought you wanted a no-ip.com account, and no-ip.com emailed you to confirm that you do want the account.
spam is like pollution in that (a) whenever you're not sure if you're doing it, you probably are, and (b) if everybody did whatever it is, life would be universally worse for, well, everybody.
Random disclaimer: Yes, we're a competitor of no-ip.com's... And yes, we used to send similar emails to people signing up for an account, although nowadays instead of sending them an initial password we send a confirm URL instead.
that's the right approach. no-ip's problem was they presumed my permission.