Easy...!
Whenever I need to obtain a login and I know the site *will* send spam, I just use a 10-minute email account...
Its set up easily enough and once you have those all important logins, 10 minutes later, the email account ceases to exist... how to keep those details for future reference? forward them to yourself and save them!
Also great for one-offs... like catalogues and brochures and the likes
My email accounts get very little spam these days