If you kiss a guy or give him signals that you like him and then decide that you don't like him follow these steps to get rid of him.

[edit] Steps

Stop flirting with him. Be more distant. Don't make eye contact or smile. Turn your head and don't let him kiss you when he tries. This includes not standing next to him.
Avoid emailing, texting, calling the guy. DO NOT SEND HIM NOTES. (This will lead him on even more)
Do not answer his emails, text messages, as well as phone messages.
Flirt with other guys in front of him. Be clear to the guy that you are not interested him, you are interested in whomeever you are flirting with. -DON'T do this if he's a friend. It will really hurt him. It's better, in that case, to tell him straight out.
You could also just straight out tell him that you don't like him anymore.
You could introduce someone new to him. Such as one of your other friends,or if you have a new bf or kinda like someone and he likes you then get him to tell the guy that you dont like him anymore if you are comfortable with doing this.

[edit] Tips

When telling the guy that you are simply not interested, try to let him down easy. Think of if you were in his shoes, and how you would like to be treated.

Be nice, and stay friends after everything cools down

Be careful not to be too mean, or else he'll hate your guts.
Make sure you don't on some level fancy him, because it's horrible when you had a chance with someone and you pushed them away. If you're not sure spend some time with the person before you make up your mind.