Biological process

down regulation of B cell differentiation, down-regulation of B cell differentiation, downregulation of B cell differentiation, negative regulation of B lymphocyte differentiation, negative regulation of B-cell differentiation, negative regulation of B-lymphocyte differentiation

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

negative regulation of B 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'.