IMESSAGE - GROUP MESSAGE

Hello, they shouldn't be able to but you can only delete them if everyone in the group is using iPhones. If just one person in the group is using an Android phone, you cannot delete them. the safest course of action is to create a new group without them in it if you are concerned.

That is a different question altogether than the one you initially asked. Removing someone does not remove the entire conversation from their device and they will have access to everything up to the point they were removed.