Dear VV,
The tax code itself cannot be deactivated nor blocked.
As suggested by the others: you can try to create a validation rule on the tax code (BSEG-MWSK1).
You find the validation in the Implementation Guide in the path R/3 Customizing Implementation Guide -> Financials -> basic settings Financials -> document -> line item -> to define validations for
posting.
Please read the documentation in the Implementation Guide.
For FI postings, you must create a validation at the moment of 2 (Line item).
You find further information on the validation in the attached note 42615, the related notes mentioned in that as well as in the R/3 of Online documentation at the following position:
FI - Financials
-> FI - Special Purpose Ledger
-> basic settings
-> validation.
The validation is originally a technology from the module "Special Purpose Ledger, therefore the description is there settled.
Additional info: you can also deactivate the tax percentage of the no longer valid tax code.
The user would then receive the error message when using this tax code, that it does not exist.
Best regards,
Vera