EDITED after @ErdemKAYA comment.
To erase a variable, use the magic command:
%reset_selective <regular_expression>
The variables that are erased from the namespace are the one matching the given <regular_expression>.
Therefore
%reset_selective -f a
will erase all the variables containing an a.
Instead, to erase only a and not aa:
In: a, aa = 1, 2
In: %reset_selective -f "^a$"
In: a # raise NameError
In: aa # returns 2
see as well %reset_selective? for more examples and https://regexone.com/ for a regex tutorial.
To erase all the variables in the namespace see:
%reset?