There are 2 features that can be used to remove signatures.
The first one is the "Signature Patterns" setting. This is a list of regular expressions (RegEx) that are used to specifically look for signatures. These work best with the "plain" "Preferred Text Type" setting and if your email come from a variety of sources, but you may need to play with the RegExes to meet your needs.
The second method is the "Text for Message End" setting. This is a word or phrase that indicates the end of the post and that it and everything after it in the email should be ignored.
For example I get a newsletter that always has the text "I hope you enjoy the post." after the real content. If I put that in "Text for Message End" everything from that text to the end of the email will be ignored.
This method works best if you have a single email source such as a newsletter.