Whenever you work with another artist, be it a producer, emcee, web designer, etc. you are hiring them based on there talent.
To a degree you have to accept there creativity as what it is.
Which means after one or two requests for revision, it's seriously time to consider cutting your loses and moving on with someone else.
But how do you tell them that you are unhappy with a product?
Be honest, tactful and nice.
Tell them that you are not feeling it, tell them why, make any suggestions you have and keep in mind you have just dissed their creation. So be sensitive to that.
This may not get you what you want immediately but it will keep you from burning a bridge and it's the mature way to handle the fact that things simply don't always workout.
Just look at it as an investment.
Wishing you continued success!