Hey @Muhammed Soyer,
There is no direct way to delete a plan via Bamboo Specs. Unfortunately there's no REST API to delete a plan as well. Could you please elaborate the requirement for doing so?
You might find this comment from my colleague useful.
However, if you are referring to plan branches, they can be made to get deleted soon after the respective branch in the default repository (configured in your plan) is deleted automatically. This is explained in Specs official documentation.
Hope that helps!