in edit->Webassist Preferences, choose memento and select Never from the list
Then close it and it shouldn't open again on its own... if it does let us know and we can open a support incident and debug and fix the problem.
Surveyor has a "remind me" option to remind you to upload a new site map regularly. You may have to clear those reminders to keep memento from opening. The option from the preferences will keep it from opening from any of our announcements, but it would still open if you asked for a reminder from surveyor.