Biological process

negative regulation of thymic T cell differentiation, negative regulation of thymocyte cell differentiation, negative regulation of thymocyte differentiation

Any process that stops, prevents, or reduces the frequency, rate or extent of T cell differentiation in the thymus.

negative regulation of T cell development in thymus

Note that immunologists typically use the word 'development' to refer to cells of B or T cell lineages undergoing the process that GO describes as 'cell differentiation'.