Not Quite What You Were Looking For
Shell Vacations keeps calling us! Is there an effective way to get them to stop?
I know this does not address the problem you're inquiring about (persistent, annoying calls) but...It might not be THE actual Shell Vacations calling. A lot of these useless travel clubs piggyback names from reputable, widely-known travel/timeshare companies in order to stop you from hanging up quickly and to sound legitimate.
Case in point. I have been a member of RCI for nearly a decade now and we keep getting these calls from travel clubs. They subtly put the words (or shall I say "letters"?) "RCI" into their name to make it sound like they are affiliated with RCI. In reality, they are just trying to get us to join their travel club and/or take timeshare tours.
Sometimes, they have claimed to be a third party that RCI hires out to contact its members. However, RCI does not do hand out members' information to other parties. Likely, a disgruntled employee of RCI or one on the take is selling/leaking members' information to these useless travel clubs. I suspect this "Shell Vacations" is doing the same - getting your and other members' information from the real Shell Vacations Club and pestering you to join their travel club.
Maybe these impostors feel that, since you own a timeshare or belong to one of these exchange clubs like RCI or Shell Vacations Club, that you would be interested in more travel opportunities.