Removing a huge group text...

My friend accidentally included in a huge group text. I deleted the text. But now when I text her that large group of phone numbers shows up. I even deleted her from my contacts and started over but that stupid group of about 20 numbers is still showing up. There is NO "i" anymore, only a plus sign??!! It's extremely annoying to not be able to delete this unwanted yet always suggested group!

IOS 11.4.1

iphone 6