If one or more people in a group conversation use Android, you cannot "edit" the group.
So you can't delete group members either.
The group messaging features only work if everyone in the group uses iMessages. Android sends traditional SMS messages, and they cannot be handled in the same way as iMessages.
The solution is to Mute the conversation. In the message overview, swipe left on the group message, hit the purple bell. You will now see a bell with a line across the group message in the overview. The message is muted, and you can then either leave it alone, or write to people in the group of which they must start a new one without you as a participant.
Leave a group text message thread on your iPhone or iPad - Apple Support