Granted that I'm not the recruiter, but:
I'm not sure I understand why you declined the offer.
If I were the recruiter and I didn't understand this, I would keep calling you.
Are you sure you have made your reasons clear? Are you sure they are clear to the recruiter?
Granted you probably wrote/said more to the recruiter than you did in your post here, more text doesn't always make things clearer.
The company was big, salary was fantastic but I didn't want to work with the technology they were using as it was very niche.
Does this mean concern about future career possibilities? Is this the only reason you declined?
Did the recruiter respond to this at all? Did he acknowledge? Did he try to defend the technology they're using and negate your opinion?
Or did he understand what you said, and clarify that there are actually other, less "niche" technologies that you will be working with?
I communicated my reasons for declining the offer to the recruiter various times now.
Reasons, plural? You only mentioned one in your post....
From a standpoint of persuading or "selling" someone, when I hear there are "many various reasons," it is often a sign of someone who hasn't really made up their mind.
If I were the recruiter or the hiring manager, all I would be convinced of by this post is that I still need to have a good conversation with you.
To get the recruiter off your back, explain your reasons. If they are really truly valid reasons to decline, they won't go away just because you talk about them. Don't leave a mystery: "Why did this guy decline? Baffling!" See if you can make the reasoning make sense to the recruiter.