User has set the rule Mark projects completed when billed 100% in Global Settings to apply on all projects by default, but want to override it for some projects, where they want to continue entering time and expenses even after the contract amount has exceeded.
You cannot override the Global Settings rule Mark projects completed when billed 100% at the project level. So you should uncheck this rule in Global Settings and use the project rule Prevent time and expense entry after reaching contract amount instead as a workaround. Adding this rule will prevent additional time and expense entries when the contract amount is reached on the projects you want. However, you need to manually set the status as Completed for the projects. Skip this rule for the projects where you want to continue entering time and expenses. Additionally, you may also want to check the rule Skip automatic time entry evaluation for such projects if you do not want Core to adjust the billable status of the time entries when the contract amount is exceeded. Check the Core Help for details.