You receive newsletters from different senders and do not want to receive these anymore? There is no blanket option to unsubscribe from all newsletters at once. You have to cancel each newsletter subscription with the actual sender.
This article describes the options for unsubscribing from a newsletter you previously requested. If you receive newsletters for which you have never given your consent, this is called spam. You can find out what you can do against spam in the article Activate Anti-SPAM in Webmail.
Write an email to the person in charge of the website
Every website of a German company is obliged to provide an email address in the imprint in order to enable easy contacting. Go to the company's website and write to the email address shown in the imprint and state your wish to not to receive any further newsletters.
Tip: As it is not legally required in every country to provide contact information in the imprint, you can determine the contact address by conducting a WHOIS query for the domain. This can be done online at https://lookup.icann.org/.