I will answer this, jabelufiroz, based solely on personal experience.......more than once, in my adult life.
I have found it necessary to back away, so to speak, or quietly "slip out the back, jack".....from a "few" friendships. Surprisingly, a couple of those friendships had been long-term.
I can assure you that a VERBAL explanation is not always necessary, nor is it advisable. It could set the scene for argument or needless high drama. When and if a relationship has reached the unfortunate place that causes you to make a decision to cease contact..... therein, is the first subtle step. Make no attempts to contact and/or communicate with the individual. In the event they attempt to contact you, in some reasonable and civil manner, make yourself respectfully unavailable and make no promises nor commitments to future discussion or activities. Make no effort toward small talk and "BE BUSY," as you add quick apologies.
In most cases, for most normal people, it does not take long nor does it require a baseball bat, for them to "get the hint."
Should this person be persistent or demand EXPLANATIONS.....Oh well, looks like you then must be upfront, specific, honest and as courteous as possible. I can promise you, this occurs rarely. Human beings KNOW when they have behaved poorly or made unacceptable/inexcusable repeated bloopers, in terms of their friends. They expect to lose friends....and don't really want or need to be TOLD, the reasons!.....Good luck :)