dnd 5e – Can a caster end a spell with no concentration, such as “Seeming”, whenever they like?

They can for a bunch of spells including Seeming, but this requires an action and does not work for any spell

There is no general rule about ending spells which do not require concentration:

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.

See more details in this answer.

Seeming specifically can be dismissed using an action, since its description says so:

The spell lasts for the duration, unless you use your action to dismiss it sooner.

Many other spells (e.g. Mage Hand) have the same phrasing and can be dismissed using an action. However, there are spells which have not (e.g. Grease). If nothing says you can end a spell prematurely, then you can’t.