Is it possible to noindex members signatures, so that any keyword used in a signature is not indexed by search engines? 

There is no way in which to do this at the present time within the software. You can noindex links (globally), but there is no way to specifically target signatures

Ok thank you, I'll turn it into a feature suggestion then. 

There's no way to tell google to not index specific parts of a page ( https://www.seroundtable.com/google-does-not-index-parts-of-a-page-independently-28021.html ) but you could disable them globally for guests in your  ACP on the Members => Profile Settings page, so that guests (and google) don't see them.

1 minute ago, Daniel F said:

you could disable them globally for guests in your  ACP on the Members => Profile Settings page, so that guests (and google) don't see them.

That's a great workaround, thank you!!