How to drop inheritance?
In addition to what @Josh already clarified, this quote from the manual should answer the question in the title:


  [...] an inheritance link can be removed from a child using the NO
  INHERIT variant of ALTER TABLE.


ALTER TABLE child NO INHERIT parent;


Either that or just delete the child table.

Also, you can include all inheriting tables when dropping the parent table using the CASCADE key word:


  A parent table cannot be dropped while any of its children remain.
  [...] If you wish to remove a table and all of its descendants, one
   easy way is to drop the parent table with the CASCADE option.


You can invert this to take advantage of the behaviour and debug your situation. Start a transaction and try to drop the parent table:

BEGIN;
DROP TABLE a;



ERROR:  cannot drop table a because other objects depend on it
DETAIL:  table b depends on table a
HINT:  Use DROP ... CASCADE to drop the dependent objects too.



The error message lists inheriting tables that block the DROP command.

Unless you get an error (aborting the transaction automatically), be sure to issue a ROLLBACK if you don't actually want to delete the table:

ROLLBACK;