Sort out the manipulators:
Basically, there are two types of borrowers. The first type contains friends who are in need of any object that you possess and you trust them to return them. Since they are good friends, their worth exceeds the worth of the object. However, if you are tired of constantly giving away, you have to deal with them nicely and tactfully without ruining the relationship. The other type contains the manipulators and social bullies. They are in a habit of borrowing things despite having the means to buy for themselves. When they do borrow, they treat your stuff roughly and are lazy in returning when you need it. You have to draw a strict line here.
Make polite excuses:
When a habitual borrower approaches you for an object that you are sure is not visibly laying around you, make polite excuses. You could say somebody else borrowed it or that it is out of order. Try to sound as genuine as you could. However, you have to keep repeating the process for every request to borrow. This will make others realize that you have learnt the trick and will not play fool again.
Keep you stuff locked and hidden:
This step is to prevent the first stage from happening which is the borrowing part. People will not ask you if they do not know you have a spare stuff. Take out only the products that you need. In school, take out only one pencil from the bag. At the boarding, try to keep only the essential cosmetics on the dressing table and keep others locked in the cabinet. This way borrowers will have no idea even if you make an excuse that you do not have what they want.
Learn to say NO:
Since they are your things and you own them, you have every discretion in the moral world to decide their use. Bring in you the courage to blatantly say NO, if you are annoyed with constant borrowing. If someone spoils a product tell them on the spot that you will not let them borrow anymore or ask for a replacement. These may seem a little too harsh but if someone does not understand with subtle hints, you need to do it. Some borrowers might even end their friendship with you but who needs such friends? People who are with you for material benefits can never be classified as friends. Prune them out.