One possibility is called a "Q-ball". A Q-ball is a finite-volume mass of bosons (net-zero-spin particles) theorized to exist by many flavors of string and quantum field theory. The bosons are not bonded to each other by traditional observations of the four fundamental forces, like gravity or electromagnetism, but instead due to a local attractive force within that mass which makes the Q-ball the most stable, lowest-energy arrangement of the particles within it. One additional theory is that this attractive force can cause the Q-ball to grow in size by capturing more bosons.
Now, the nucleus of a Helium-4 atom is a boson; the net spin of its 4 fermions is zero. Helium-4 nuclei could therefore be attracted to and captured by a Q-ball that wandered into the mass of a star. Bosons within the Q-ball would not undergo interactions like nuclear fusion, thus robbing the star of a key late-stage fuel and fusion source. This would affect the star's density and thus its temperature, disrupting the stable dynamic equilibrium of the star's interior environment, potentially slowing hydrogen fusion and creating a vicious cycle that, in theory, could eventually "kill" the star.
This was one theory proposed by the writer of the movie Sunshine, as a scientific basis for why the Sun's fusion was prematurely slowing (causing a reduction in solar radiation, and catastrophic global cooling of Earth, prompting two separate expeditions to deliver a vaguely-described device to "restart" the Sun). However, the Sun's mass is insufficient to trap a Q-ball, and our best estimates of the Sun's age indicate only 25% of its mass would be helium, with minimal helium fusion occurring, so even if a Q-ball were in the Sun, there's nothing significant it could inhibit for billions of years. And, should our Sun ever start to behave abnormally, there's nothing at all the human race could do with current or near-future technology to influence it; the entire mass of Earth is 1/333,000 the mass of the Sun, so anything we had available to use to build any such device would be a grain of sand on a beach relative to the raw energy of a star or anything that could possibly actually influence it.