How should one respond to 'Call for Paper' e-mails?
Every conference and workshop will issue a call for papers. The editors of special issues of journals may issue a call for papers. Editors of books on a special topic may have call for papers. How else will they advertise their conference, workshop, journal, book and get submission?
Many of these will be perfectly legitimate and even highly reputable venues. Naturally, some of these might be poorly ranked conferences or even in-it-for-the-money ventures, rather than serious scientific venues. But if you mark them all as spam, how will you know what is out there to submit your papers to?
The best strategy I can think of is to create a folder to collect these call for papers and use a filter to move them into that folder. When you need to submit a paper, you can search through the folder to find an appropriate venue. As most conferences repeat yearly at roughly the same time, you can collect the names of those and the approximate due dates in another file (or calendar), so that you build a good picture of what the major events in your field are.
Every now and again you will need to find a new forum or perhaps there will be a book that you could contribute to. By filtering all call for papers, you miss these things.
On the other hand, you can find most of this information using Google ....
Answering the title question: generally you shouldn't respond to these emails. That said, you could send an email to be asked to be removed from the mailing list, but this is probably ineffective.
Unsolicited CfP emails are a very bad practice, but there is little you can do. Just ignore and delete the irrelevant CfPs manually.
Reputable conferences and journals usually send their announcements only to the relevant mailing lists, so that only those who are really interested in the CfPs will receive them. Unfortunately, there are some reputable venues that resort to spamming, and there is also the grey area of people forwarding CfPs to their colleagues, departmental mailing lists, etc.
Automated filtering sounds tempting, but it is very dangerous. For example, an invitation to serve in the programme committee of a conference may look very similar to a CfP. Such invitations often include the same basic information as a CfP (conference name, location, date, topic, web site URL, chairs, etc.) – indeed, it might even include a preliminary version of a CfP. An automated filter might easily classify it as CfP spam.