How to completely disable scaling in App Runner?

I apologize, but it's not possible to completely disable scaling in AWS App Runner. App Runner is designed with automatic scaling as a core feature, and there isn't a way to turn it off entirely.

Even when you set the minimum and maximum sizes to 1 in an auto scaling configuration, App Runner may still temporarily scale up to handle traffic spikes or during deployments. This behavior is part of App Runner's design to ensure high availability and seamless updates.

For your specific use case where scaling causes issues with your service, App Runner might not be the best fit. You may want to consider alternative AWS services that offer more control over scaling and instance management, such as:

1. Amazon ECS (Elastic Container Service) with a single-instance task definition
2. Amazon EC2 (Elastic Compute Cloud) where you can run a single instance
3. AWS Elastic Beanstalk with a single-instance environment

These alternatives would allow you to have more precise control over the number of instances running your application, which seems to be crucial for your service's proper functioning.
Sources
App Runner scale-down behavior | AWS re:Post
Managing App Runner automatic scaling - AWS App Runner
based on what metrics does the AppRunner service scales up or down? | AWS re:Post