In the PHB it says that if a melee attack you make reduces an enemy to 0 hitpoints, you can chose to knock them out. My question is, is there any way to knock out an enemy with out hurting them to the point of death? I mean as in punching an opponent in his temples, hit him w/ a club, etc. I am not including spells like sleep, etc.
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Not using spells or magical items, or magical effects, a creature becomes Unconscious if:
Knocking a Creature Out
The key here is the Condition: Unconscious versus Incapacitated. (Stunned shown for comparison at the end). Not all Incapacitated creatures are Unconscious, but all Unconscious and Stunned creatures are Incapacitated. (See Conditions, Appendix A, Basic Rules)
If you don't mind them being awake, a few other things will Incapacitate a creature:
Stunned probably won't fit your requirement as it is very short term.
Stunning Strike (Monk)
Oytugh special attacks can similarly stun a creature.
To answer your question from a rules-as-written perspective, no. There are no rules for sapping, choking, cold-clocking, etc. that would allow you to bring an enemy to unconscious without first dropping their HP to 0.
I should point out something, though: choosing to knock a character unconscious at 0 HP instead of letting them go to the dying stage is specifically not hurting them to the point of death. In D&D 5e, characters don't die when they reach 0 HP, and they don't even start dying unless their attacker wants them to die at 0HP.
Otherwise, it's pulling the last punch so they safely fall unconscious at 0HP. This does not, however, prevent them from going into dying if they take further damage.
On the other hand, there may be other ways. For example, Drow (the 1/4 CR monster entry from the MM, pg 138 maybe?) have a poison effect on their crossbow attacks that knocks the target unconscious for an hour if the CON save fails by more than 5. I'm sure there are ways for PCs to get poisons that behave similarly. As with many things in 5e, this is more in the realm of "ask your DM" than it is in the realm of hard rules.
Certain Poisons Do the Job
"Sample Poisons" on DMG p.257 has two possibilities: Essence of Ether (inhaled) and Oil of Taggit (contact) cause their victims to fall unconscious. At 300gp and 400gp per dose, RAW they are rather spendy ways to accomplish the task. It would be straightforward to house-rule a less expensive version of Essence of Ether that required, say, an extra Grapple check to place a ether-soaked cloth over the victim's face.
From the earliest days of D&D this has been a problem. Hit points prevent you from dying, and an unconscious character can be quickly killed so from a player's defensive point of view there should not be any easy or common way to circumvent hit points by knocking you out.
Of course this runs counter to any amount of source-material examples where being knocked out is a prelude to some exciting confrontation or escape scenario that the writer wants to unveil. Even Conan gets knocked out from time to time. The author then protects the character by inventing a reason to keep them alive.
But because the writer (the DM) is not supposed to be in control of all the characters (the PCs) in a role-playing game, this is not easy to carry over without railroading and the classic example in D&D used to be Against the Slave Lords, a hugely divisive module series which has exactly this plot device in it.
Again, from an offensive PoV, if there is any simple means of knocking out opponents you can bet that any sensible group of players will try to do it all the time, at which point their opponent will have its/their throat slit and any hit point total rendered useless.
So it's a very difficult design challenge to find a mechanism which works when it's fun and exciting, but can't be used when it wouldn't be. Aside from anything - fun and exciting for whom?
I know this is not really an answer to the direct question, but I hope that it shows the reason that knocking things out is something that is never going to be easy in D&D, or any RPG that places a high value on player agency and deadly combat, and why you should be very careful about introducing such a thing lest you break your campaign.