"I don't like you"
Translation
Informal
To him & her: Tu mahnne pasand koni.
Respectful
To him & her: Aap mahnne pasand koni.
BONUS!!
If you want to make an impact and truly mean what you are saying, then you should try this:
"I don't like you at all"
Translation
Informal
To him & her: Tu mahnne bilkul bhi pasand koni.
Respectful
To him & her: Aap mahnne bilkul bhi pasand koni.
(Cheesy picture courtesy: town in Rajastan)