The central ethic within Wicca is, "An ye harm none, do as ye will."
This applies not just to spells and rituals, but also to everyday life.
For many years, Witches have been cast out as being "evil" people, or we have wrongly taken the blame when something goes wrong. Yes,even in todays society,ignorance knows no bounds.
So, I believe that it is up to us to break those myths.Anything causing harm to anyone, including oneself, and anything that limits any ones free will should be expressly avoided!
What does all this mean, well the obvious is, no you can't put a curse on someone.
The not so obvious is, no you can't MAKE someone fall in love with you.
Doing this would limit that persons free will and may inadvertently harm another (like his girlfriend, and I don't care what a b***h she is)
So, how can this be avoided? Well, let's say that all the person wants is to find a romantic partner and that in their mind, this person is the best choice. Why not do a spell for "A lover who is perfect for me." and put the energy out into the universe to bring whoever that is into your life.
If you are planning on doing a spell that may or may not be ethical, how can you figure out weather or not to do the spell? Well, one good idea is to think it over.
What is my specific need or want?
Are there any other ways to go about getting it?
Another good idea, if you know anything about tarot or runes is to do a reading of some sort to see what the results (or consequences) of doing such a spell would be, and to take the results of the reading into consideration before performing the spell.