Can I resign if my opponent only has his King? Well, obviously if my opponent has only a King, I cannot lose...either a win or a draw. Suppose I also have a Queen(Forced win).

However suppose I want to lose (maybe I am winning and want to grant some points to my opponent or whatever), so can I resign under such a circumstance ?

(I understand that my opponent would have resigned before reaching such a position, but anyways is it legal to resign if opponent has only king? )
The FIDE laws of chess contain two parts: Basic rules of play (Sections 1-5) and Competition rules (Sections 6-12).

The first part is what defines how the pieces move, when the game ends, and so on. These rules indeed allow players to resign at any point before the end of the game, so you can resign right before you would give a checkmate, but not after you have released the piece after the final move. The second part explains things such as the chess clock, scoresheets, the role of the arbiter etc. Curiously this part seems to have nothing to say about deliberately losing. 

So technically, one could say that resigning in a won position on purpose "is allowed" by the rules of chess.

However, the FIDE tournament rules, which are used in international tournaments, say that


  8.(f) Where it is clear games have been pre-arranged, the CA shall impose suitable penalties.


I don't have a good credible source, but I think that deliberately losing games (which has happened) are usually considered in a similar way. In addition, deliberately losing games is generally considered unacceptable behaviour in sports [citation needed]. So in my opinion, it is clear that deliberately losing a game is against the rules of a tournament (unless explicitly specified if the tournament organizers want to allow such tactical manoeuvres). You can always ask the tournament organizer if you want confirmation.