First, that was me who just applied to your group, you can reject it if you want, I was going to leave anyway; I wanted to see how it's setup regarding new member apps.
IMO you'd be better off creating and enabling a pending membership message that applicants have to answer (or no approval otherwise), and that will drastically cut down your spam/scam filtering work, as the large majority of those folks use bot software for their spamming/scamming, and you usually don't get back replies to the pending membership question, so they can bot-apply all they want, since they will never reply to the pending question email, no approval. You can then leave them on the pending queue until they are automatically-removed in 14 days (unless if you claim them)
You can write the pending membership question email as such to where if one replies you'll be able to tell if it is a real person, and also whether they are legit or spammers. On all the groups I own & manage we have that and it rarely allows a spammer to get it. Here's an example of what I use:
Hello and thank you for your interest in the Airliner Modelling Digest group! (AMD@groups.io)
We only have a few basic rules, one of them is to hear from you before accepting your membership application, to make sure you are a legitimate person/modeller/hobby store/decal maker/researcher/etc, and not some bot or spammer!
So please reply to this email and in a sentence or two, please let us know why you'd like to join our group.
The Group Admins & Mods
And if their reply is not quite-clearcut and you suspect they may be legit, you can always send them a second email and ask them more group-specific questions, like which generator model they have, what's their setup, or anything else that will tell you pretty-much they are real and legit.