unrequested former member approval on club website

I recently got an email indicating that former club member had requested approval to log in to our club website (5481.toastmastersclubs.org).  When I contacted him to verify his intent, and to invite him to rejoin toastmasters before getting on our website, he told me it wasn't him, but likely a spammer.

Today, I found another email telling me he had approved his addition to the club website.  I find no indication, logged in either as admin or as myself, that he has logged on, or has been added back onto our site.  I'm a little alarmed that I should receive such an email, because it appears to be from a hacking attempt, and I don't see any evidence that it's legit, or that the addition or logon has actually happened.

What can I do to prevent or block this?

I'm guessing you are referring to former member Luis.  He shows as a recently deleted prospect and as a former member - with 2 different email addresses.

The recently deleted version was added to the system about a week ago - 4 minutes after Marti accessed the system.  Did Marti add any new prospects about a week ago?  That would have sent an opt-in message to Luis (or his entered email address), which has subsequently been accepted by the person at that address. 
Presumably you or another officer has subsequently deleted that entry

Pam,

Turns out the person in question who I thought might be a spammer was actually an interested new member with the same name as a former member.  Hence, this thread does not represent a problem after all.
Thank you for your assistance.  The information you gave in your reply was exactly what we needed to iron the potential problem out quickly and easily.

Best Regards,

Dave