There isn't a setting I know of to disable rebuild.
There is an options setting to lower the performance impact though:
raid.reconstruct.perf_impact Sets the overall performance impact of RAID reconstruction. When the CPU and disk bandwidth are not consumed by serving clients, RAID reconstruction consumes as much as it needs. If the serving of clients is already consuming most or all of the CPU and disk bandwidth, this option allows control over how much of the CPU and disk bandwidth will be taken away for reconstruction, and hence how much of a negative performance impact it will be to the serving of clients. As the value of this option is increased, the speed of reconstruction will also increase. The possible values for this option are low, medium, and high. The default value is medium. There is also a special value of default, which will use the current default value. When mirror resync and reconstruction are running at the same time, the system does not distinguish between their separate resource consumption on shared resources (like CPU or a shared disk). In this case, the resource utilization of these operations taken together is limited to the maximum of their configured individual resource entitlements.