Haven't noticed such a setting but then there are probably many I haven't.

If you're happy to code a bit you can add {rel="nofollow"} to the Link script in the plugin

See http://rogerlovejoy.net/blog/b2evo.php/add-target-option-to-link where I modified the _quicktags.plugin to add a target option to my links

Update

OK until someone shows you a setting; you can ~ in version 6.9.3 /plugins/_quicktags.plugin.php

Change Line 451 to

b2evoButtons[i].tagStart = '<a href="' + URL + '" rel="nofollow">';

Thanks for your solution and in the meantime I've looked around B2evolution and found the option "rel = nofollow" in the AutoLinks.

The solution I have published in my German blog at https://b2evolution.internetblogger.de/2017/11/15/in-cms-b2evoltuion-mit-nofollow. So you can see on the screenshots what I meant. I knew that there was this Option, but then not with TinyMCE editor tooling.

Your solution certainly works too. Thanks for the good cooperation :)