  The existence of substructure in halos of annihilating dark matter would be
expected to substantially boost the rate at which annihilation occurs.
Ultracompact minihalos of dark matter (UCMHs) are one of the more extreme
examples of this. The boosted annihilation can inject significant amounts of
energy into the gas of a galaxy over its lifetime. Here we determine the impact
of the boost factor from UCMH substructure on the heating of galactic gas in a
Milky Way-type galaxy, by means of N-body simulation. If $1\%$ of the dark
matter exists as UCMHs, the corresponding boost factor can be of order $10^5$.
For reasonable values of the relevant parameters (annihilation cross section
$3\times10^{-26} ~\textrm{cm}^3~ \textrm{s}^{-1}$, dark matter mass 100 GeV,
10% heating efficiency), we show that the presence of UCMHs at the 0.1% level
would inject enough energy to eject significant amounts of gas from the halo,
potentially preventing star formation within $\sim$1 kpc of the halo centre.
