I have and the only person that ninjamances me is Alistair even when I only gain approval through gifts and actions and don't even speak to him. I wish Leliana was who I had to worry about might feel less like kicking a stubborn puppy then.
Do you play on the PC? If you do, there's a mod that restores some of the dialog that actually let's you be a nice person when turning down his advances XD (one conversation that stands out is when you give him his mother's amulet, and he's all "You remembered??" and your original three options are "Of course you're special to me!/ Let's not make more of this than it is..../ Sorry? Did you say something?". The mod adds something like "Well yeah we're friends", which should have been there in the first place imo
)
With Leliana, I've never been ninjamanced either. You just accept that compliment and tell her she's a good friend and she never hits on you again (that's been my experience). I rolled a Warden for her, and it was interesting in that I felt like I had to be kinda blunt when romancing her - like I asked her if she "often enjoyed the company of other women" and then I think I said that my warden did too.
It may be different for the male warden though - it's been years since I played one, and he was all for Morrigan anyway XD
Anyhow, if you go back and play Origins anytime, hope that helps ^.^