loser (+unattractive) guy = low self esteem
low self esteem (can)= clinginess
if i'm a loser, and i have low self esteem, and some attractive girl talks to me (even if it's just to say 'hi') instead of brushing me off like I don't exist, then i'm going to follow her around like a lost puppy until she does tell me to get lost.
all of us losers want to date attractive women, and by you being nice, it makes us think that we have a chance... i'm not saying you have to be a bitch, but if you want to drive away the losers, then you have to stop putting out the 'I'm available to anyone vibe'... if you want to find an all around good guy, you gotta raise your all around standards. Stop trying to convince yourself that you should 'settle' for someone you're not physically attracted to. Go for someone that is just as or nearly as attractive as yourself. (birds of a feather) that's not the only quality to consider of course, but it's an important overall factor.
there's a power base to be considered in every relationship. you should try n avoid being both better looking and smarter than any guy that you date. Choose one or the other or it's gonna throw everything off balance. It sounds like you're totally cool w/ dating someone that's not as attractive as you, but you haven't mentioned intellect. clearly the last few wannabe suitors haven't been very bright.
anyhoo... i could go on and on, but i'll stop here. Just remember that you're putting out a vibe (albeit subconsciously) that you need to stop. You deserve someone great, so expect it... insist on it... and don't even think about settling for anything less.