Is it possible for an active volcano to become less violent over time?
The ultimate fate of all active volcanos is to become extinct volcanos, and eventually, objects that are barely recognizable (or not recognizable at all) as extinct volcanos.
This process from pre-volcano to active volcano to extinct volcano to barely recognizable extinct volcano takes a long time, even by geological standards of &quot;a long time&quot;. The oldest elements of the Hawaiian–Emperor seamount chain predate the Chicxulub crater, and even those oldest elements are easily recognizable as extinct volcanos. Active volcanos are active for only short periods of time (short by geological standards), a few hundred thousand years at most. That of course is a very, very long period of time by human standards.

Alternatively, would it be possible to terraform the volcano to make its eruptions less dangerous?

Consider the follow hypothetical doctor-patient conversation: &quot;Doctor, it hurts when I do this!&quot; (The patient then bonk himself on the head.) The doctor's response is simple: &quot;Don't do that then.&quot; The easiest way to do this with regard to active volcanos is to stop having humans live near active volcanos. What is feasible is to vastly improve the ability to predict when volcanos will erupt. The state of the art in this regard is rather lousy (the state of the practice is lousier still),
That is not feasible; there are too many humans who already live near active volcanos. Terraforming also is not feasible. While the amount of energy released by volcanos per year is a lot less (orders of magnitude less) than the amount of energy the Earth receives from the Sun, it is still orders of magnitude higher than the amount of energy consumed by humanity. To make matters worse, that human consumption is widely dispersed while volcanic energy is concentrated in a few places.