Since this has been here a while I assume you fixed this. If not, as you are probably aware, this site offers an excellent jammennaps response for everything except Chrome OS. I just searched "chromebook jammennaps" and found a number of chrome specific mitigations. Which ones would be the most effective choice are beyond the scope of this response. I personally would just carefully back up my data (scanning for lurking malware) and do a clean OS reinstall. I found this in the long run to be the easiest solution to problems of this sort taking into account time, effort, and ending up with a system you are assured is clean. Well, except for persistent BIOS-level malware attacks. But forget I said that.
Edit: I stand corrected. Chrome is included in here.
This guide teaches you how to remove the Jammennaps.com pop-up ads virus by following easy step-by-step instructions.
malwaretips.com