I never understand people who do this - terribly inconsiderate and frustrating.
When is the event - next weekend? Unless it is a wedding or a big black-tie gala, seven weeks out may be too much time (not that that excuses the behavior, but that's a long time for an RSVP window).
I think one reminder/request is sufficient. If you still don't hear from them, assume they aren't coming. If there is a seated meal, do not set a place for them - no place card, no name tag, etc. If it is more of a cocktail function, again, no name tag, but it's less of a big deal if they show up - the venue can't be too upset over 3-4 more people (or less; some who said 'yes' may not show).
It's not very polite of me, but I've been known to greet people who showed up without RSVPing with a "Oh, I wasn't expecting you! Nice to see you. Excuse me - I see the cat's on fire."