I am using "write to data store" smart service. How I can capture erro

Out of the box, you cannot capture error/faulty messages in write to data store service.
However, as an alternative you can plugin your write to data store node in a sub process and put an exception on this sub process node with a timer.

If there is any error in the node, the exception will execute and you can identify that and can drive your further business logic.

by default it sends alert. how to disable that ?

You can configure the alert group by going to the Alert tab of the Process model property, and can specify the only user/group which should be notified when there is any error/alert.