@Gosavi, Pravin Thanks for posting your question in Microsoft Q&A, apologize for any inconvenience caused on this.
Unfortunately, there is no way to stop the app service plan. Even if you stop all the webapp inside the app service plan you will be charged as per the pricing mentioned here based on the SKU size of the app service plan.
The only suggestion/workaround to reduce the cost saving is to downgrade the app service plan to either Free or shared tiers during non-working hours.
Hope this helps, let me know if you have any further questions on this.