Use find for name "a" and execute rm to remove those named according to your wishes, as follows:
find . -name a -exec rm -rf {} \;
Test it first using ls to list:
find . -name a -exec ls {} \;
To ensure this only removes directories and not plain files, use the "-type d" arg (as suggested in the comments):
find . -name a -type d -exec rm -rf {} \;
The "{}" is a substitution for each file "a" found - the exec command is executed against each by substitution.