  The ability to control microwave emission from a spin ensemble is a
requirement of several quantum memory protocols. Here, we demonstrate such
ability by using a resonator whose frequency can be rapidly tuned with a bias
current. We store excitations in an ensemble of rare-earth-ions and suppress
on-demand the echo emission (`echo silencing') by two methods: 1) detuning the
resonator during the spin rephasing, and 2) subjecting spins to magnetic field
gradients generated by the bias current itself. We also show that spin
coherence is preserved during silencing.
