  Stellar-mass black holes with relativistic jets, also known as microquasars,
mimic the behavior of quasars and active galactic nuclei. Because timescales
around stellar-mass black holes are orders of magnitude smaller than those
around more distant supermassive black holes, microquasars are ideal nearby
`laboratories' for studying the evolution of accretion disks and jet formation
in black-hole systems. Although studies of black holes have revealed a complex
array of accretion activity, the mechanisms that trigger and suppress jet
formation remain a mystery. Here we report the discovery of a broad emission
line during periods of intense hard X-ray flux in the microquasar GRS 1915+105,
and highly ionized narrow absorption lines during softer states. We argue that
the broad emission line arises when the inner accretion disk is illuminated by
hard X-rays, possibly from the jet. In contrast, during softer states, when the
jet is weak or absent, absorption lines appear as the powerful radiation field
around the black hole drives a hot wind off the accretion disk. Our analysis
strongly suggests that this wind carries enough mass away from the disk to halt
the flow of matter into the radio jet.
