There has to be a way to block iPhone group text that are only solicitation

My wife, myself and now my Grandson get these sexual invite text. They come from multiple numbers. I've blocked the numbers one at a time but then whomever sends the text adds a number. My wife and I just got tired of trying and just delete them, but now my grandson is getting them and there has to be a way to block group text that are only solicitation for some dubious reasons. Getting into an Apple store for this has been a hassle so I'm reaching out to all of you for an option. They come from like 20 numbers, we block all the numbers and then select hide alerts but then we get them again and a number is added and we block that number and get it again with all the blocked numbers but a new number is added?

How do we get out of this unwanted sexual thing?

[Re-Titled by Moderator]