Most ISP's do not release their user lists, but some less than honest ISP's sell their user lists for extra bucks on the side. Once your email address joins a spammers list, you may get hit with dozens of junk emails a day as many dedicated spammers share their lists with each other, or are really agencies representing many dozens of companies and individuals, each trying to target you with their product. Unfortunately, the Internet is not as secure as some would have you think.
Some spammers play 'cat and mouse'.
They repeat this as often as is possible or necessary to keep their spam flowing. Some 'clever' spammers claim to offer a removal service. In reality, this is simply 'sucker bait'. By replying, you are in fact confirming to the spammer that your email address is a valid place for them to send their spam to. Unfortunately this type of spam is now fairly rare, due to the ease of tracking the spammer.