Declaring Star War(s)
It's common knowledge that if Earth is obstructing your view of Venus Illudium Q-36 Explosive Space Modulator will solve the problem with an Earth-shattering ka-boom. But what if that star in a neighboring system is in your way? Maybe some pesky creatures are on a planet orbiting a star you want reduced to atomic dust blowing in the solar wind.
You declare star wars by which I mean you weaponize a star to explode.
The most straightforward way regardless of the star type is adding massive amounts of iron into the star's mass. It takes more energy to fuse iron atoms than is released in the process so you're essentially poisoning the star with iron.
Another great way to do interplanetary pest control is dumping a massive amount of hydrogen onto a white dwarf to make it flare.
Where do you get massive amounts of iron and/or hydrogen to commit planetary genocide? That's simple. Find a massive star you want to extend the life of. Heavier elements are a waste by-product of fusion and needlessly tap away the efficiency of a star. Huge magnetic fields can lift iron and other metals out of a star's core. That's tricky since a star can generate opposing magnetic fields. Metal atoms which the star would otherwise use energy trying to fuse into even heavier atoms are there for the taking.
There is a whole other Isaac Arthur video about star lifting. I'll probably get to that eventually.
As the video points out more massive stars tend to burn through their fusionable fuel faster than main sequence stars. A technologically advanced civilization could siphon hydrogen off a massive star and bring it into the main sequence. That civilization could then use the siphoned off hydrogen as fuel in artificial fusion reactors or compress it until it undergoes fusion to create two or more smaller stars than its parent.