We all have one, know one or are currently hiding from one - a mere acquaintance who wants to be more, the mother-in-law who never stops calling, or maybe the old high school friend who thinks you are still back in 9th grade and wants to spend every waking moment reliving the past with you.
My son, at the tender age of 9, made the mature observation that there were just some people in the world who could drain you - who could as he put it, "pull the life right out of you" and dubbed them "ensucklanators". I thought it perfect, and even though he is now 25, if he says "Oh, no Mom, another BIG E," I know exactly what he's talking about.
I really don't think the same method will work for them all. They are as varied as the stars, but the point being to preserve life - your own (the ensucklanator can cause untold emotional death), and you must understand that you may not be able to preserve any kind of relationship with this person and in the end, they may exit your life altogether.
I think it's harder for immature people to handle those who would encroach on their space or their lives. They are so concerned still with being polite and not hurting feelings. But if in being only assertive it happens anyway, it's something that can't be helped.
We must maintain an attitude of ownership where our own life is concerned, maintain the right to choose how and with whom we spend time and resources and then not waver back and forth weakly, sending mixed messages. Most of our own frustration with those who would irritate us is our own fault, a product of a failure to maintain personal boundaries and to set limits with people.
Nothing anyone tells you, no pat answer or word for keeping these people at arm's length, will work if you cannot assert yourself, and if you can, you need nothing more. I always make myself count to three before answering any request, just to prevent the automatic throwing around of "yes"es to things I may regret. Most energy is wasted by overextending ourselves, saying we will when we would rather not, spending time with those we don't enjoy, doing things we don't delight in and feeling we somehow owe someone that we don't.
If you find that you are trapped in this kind of pattern, nodding yes and regretting it almost instantly, practice saying "No" to people. Get a close friend to help or do it in the mirror. Then start small and work up. Remember people who love you can hear "No", and that we do teach others how to treat us by what we allow. If you learn this one, they will, too, and soon the requests will not come so often, as they will learn you are not an easy touch, sitting around waiting for them to need you.
If the problem is people that happen upon you (I had a neighbor once that caused me to peek from windows before leaving the house), who engage you in long-winded conversations you can't extricate yourself from, you aren't standing tall enough. Putting a hand up in front of you, smiling sincerely and stating, without apology or qualification (which gives them a foot in the door to convince you otherwise), "I can't talk now, I'm on my way out. But hey, enjoy the gorgeous day," and then SCOOT, not giving them time for a comeback. If they are someone you just can't be that direct with, or someone you do want to talk to just not right now, throw in an invite to call you later and make a date to talk with them.
I am famous for my hatred of "The Uninvited Guest". Even my closest friends know not to drop by without warning. I'm just not cordial that way. I want notice, and even five minutes is enough, but I need a chance to say yes or no. I want a choice when it comes to company. I had to teach many this by not letting them in and saying "OH, if you had called first I could have told you that I'm busy. When could you come back?", and saying it at the door.
Then there are those who never get it, and you can be pretty direct and still, it's not sinking in. But it all comes down to maturity - being able to stand up for yourself, being able to sit a person down who is imposing and telling them directly, kindly and with no wiggle room that you can't abide by their methods of interaction with you. I lost a friend this way once, but it had to be. When I explained to her that I just didn't have time to spend so much with her, that I liked her but that my life was so full already, she only heard that I was rude and mean and she left crying. But that's going to happen with some people and at least she isn't on my door step with coffee every morning now, thinking we are best friends.
So, it's really up to you. Learn to speak your mind with conviction, with authority and with confidence in what you are saying and you can handle any old ensucklanator. Don't learn that one skill, and be at their mercy forever, giving your time, your attention and your very life to people and things that don't matter to you. Life is just too short, too precious for that.