Spontaneous emission through a coupled cavity can be a significant decay channel for qubits in circuit QED. We present a new circuit design that effectively eliminates spontaneous emission due to the Purcell effect while maintaining strong coupling to a low Q cavity. Excellent agreement over a wide range in frequency is found between measured qubit relaxation times and the predictions of a circuit model. Using fast (nanosecond time-scale) flux biasing of the qubit, we demonstrate in-situ control of qubit lifetime over a factor of 50. We realize qubit reset with 99.9% fidelity in 120 ns.