It can also do PMU control when the machine is 'turned off'. The alternative is to use an external microcontroller. It is actually quiet useful.
What is your reason for wanting to disable it?
Those core execute the codes from their local SRAM which can only be written to under very specific conditions. You can't arbitrarily write to their SRAMs.
An SOC has various bus arbitrators that are built into hardware which control the dataflow. It is part of the chip's backbone. I've never seen an architecture in which you could easily write to the aux core's SRAM. This is partially because those cores are often responsible for DRM therefore access to them is very restricted, but also because it is expensive (in terms of gates) and unnecessary to hang them off the main bus.
It is a very unlikely "DMA hacking" vector.
The peripheral can just as easily DMA to main memory and overwrite kernel code if the memory apertures are set wide open to allow peripheral to DMA anything into the host. Additionally unless you have PCIe or a similar bus with mastering capability a peripheral can't DMA.