We have provided the option of A/B testing to test which subject lines are getting you better results. But when you identify the variant that is not working well, you would have to delete the variant in the step. One downside with this is that it deletes the stats for these variants from you reports.
We have now introduced a way to activate/deactivate a variant in a step whenever you want by selecting the toggle adjacent to the variant. When a variant is deactivated, the remaining prospects in the step are automatically moved to the active one and emails continue to go.
Note: You cannot deactivate all the variants in the step at a time. There should be at least one active variant in the step.