I've never heard it used as an idiom, but it sounds as if you are comparing people who habitually take something from you to leeches and should avoid them because they would keep taking from you -- perhaps energy or money.
keep it under the hat
It means to keep them unaware, to keep them from finding out the truth.
A white elephant means 'something useless and expensive to keep'
"Keep up the work" really isn't a common idiom, so I don't have another idiom that means the same. Sometimes you hear "keep up the good work," which means to continue doing a good job, to keep on with the quality you're doing now. However, I still can't think of another idiomatic expression that means the same thing...
An excellent expression. To keep an eye, or to keep an eye on things means to maintain a watchful eye. You monitor what is happening with an appropriate zeal to the occasion. Another idiom would be, to keep an eye peeled. It means you are watching and protecting something, or someone.
The expression "Keep it dark" refers to telling someone to keep a secret. The phrase uses the imagery of staying in the dark as something that should remain unseen.
Nothing - however, keeping a person in the dark means to withhold information from them so that they are unaware of the truth, as if they are stumbling around in the darkness trying to find it.
"Bear in mind" just means to keep something in your mind. You could say "bear in mind that this answer might be wrong," or "bear in mind that the weather might change."
The idiomatic meaning of keep the pot boiling means to to keep it going actively.
It is a horse-racing term. If you keep control of a horse's head, you control the way that the horse runs, and how fast it goes. Keeping your head means keeping control of yourself and staying calm in all situations.
Think of someone trying to get out of their backyard and trespassing on someone else's yard. This phrase means "mind your own business and stay out of mine." It's an emphatic order to keep to yourself.
"Zip your lips" is an idiomatic expression that means to stay quiet or not to speak. It is often used when someone wants another person to stop talking or to keep a secret.