Biological process

down regulation of memory T cell differentiation, down-regulation of memory T cell differentiation, downregulation of memory T cell differentiation, negative regulation of memory T lymphocyte differentiation, negative regulation of memory T-cell differentiation, negative regulation of memory T-lymphocyte differentiation

Any process that stops, prevents, or reduces the rate of memory T cell differentiation.

negative regulation of memory T cell development

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'.