Concentration spells can be dispelled at will
Simply dropping concentration (which takes no action, per the rules on concentration) ends the spell prematurely, unless the spell description overrides what happens when concentration ends (as conjure elemental does).
If the spell is not a concentration spell, you can end it early if the spell description says so, but there is no general rule allowing non-concentration spells to be ended early. (Note that the Duration section of the spellcasting rules does not mention being able to end non-concentration spells early.)
The Sage Advice Compendium is an official FAQ document for Dungeons and Dragons, fifth Edition. You can check out version 2.3 here for free. On page 13, this specific question is answered as well:
Can a spellcaster dismiss a spell after casting it?
You can’t normally dismiss a spell that you cast unless (a) its
description says you can or (b) it requires concentration
and you decide to end your concentration on it. Otherwise,
a spell’s magic is unleashed on the environment, and if you
want to end it, you need to cast dispel magic on it.