Dependent Data Validation Lists and the rule of no spaces in named ranges

You may include another feature to the dependent data validation list.

Like when you change your selection in A2, the dependent drop down list in B2 still contains the old selection if it was selected earlier. So you may apply the conditional formatting to hide the previous selection if any in case you change the selection in A2.

Formula used to make a New Rule for conditional formatting for the cell B2 is....

=COUNTIF(INDIRECT(VLOOKUP(A2,vlookup_table,2,FALSE)),B2)=0

And custom format B2 with ;;; through conditional formatting.