Biological process

down regulation of lymphocyte differentiation, down-regulation of lymphocyte differentiation, downregulation of lymphocyte differentiation

Any process that stops, prevents, or reduces the frequency, rate or extent of lymphocyte differentiation.

negative regulation of lymphocyte 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'.