Erin Stellato walks us through the process of removing a database from an Availability Group and then re-adding it later:
I recently had a scenario in a two-node Availability Group where multiple large-batch modification queries were executed and created a large redo queue on the replica. The storage on the replica is slower than that on the primary (not a desired scenario, but it is what it is) and the secondary has fallen behind before, but this time it was to the point where it made more sense remove the database from the replica and re-initialize, rather than wait several hours for it to catch up. What I’m about detail is not an ideal solution. In fact, your solution should be architected to avoid this scenario entirely (storage of equal capability for all involved nodes is essential). But, stuff happens (e.g., a secondary database unexpectedly pausing), and the goal was to get the replica synchronized again with no downtime.
Click through for the demo.