Objects (like comb)can be charged by rubbing as charged particles, particularly, electron are transferred from one object to other. This can be seen as object (comb) attracts small bits of paper. After sometime, charge on body seems to disappear. How does this charge disappear without any external influence ?
Of course there is. You have the air around you, don't you? Nature does not particularly like a buildup of charge, and will try anything to get rid of it. In physics terms, a buildup of charge will have a high value of electric field nearby ($\rho=\nabla\cdot\bf E$ of course), and this large electric field will lead to the charge getting forced out.
Insulators aren't conducive to the motion of charge, they tend to get charges embedded in them. Yet, charge still can move--and thus leak off into the atmosphere. Once there, it will automatically spread out and subside.
The charge may also leak a bit to the interior of the insulator--but this won't change anything.