get account activity in postgresql
I do not find any system table show the  last login  time of roles。 if you want to see the 
last login time of a role and the executed command of a user ,I suggest you set log_connections 
parameter in postgresql.conf ,and you can see those information in your database log files.

In addition, you can use "\s" meta command see history command ,as the following:

[pg90@redhatB ~]$ psql
psql (9.0.9)
Type "help" for help.

postgres=# \s /home/pg90/script/tf/history.sql
Wrote history to file ".//home/pg90/script/tf/history.sql".


But only can see the history of one client.

To see other roles's connections information ,you can see pg_stat_activity for more informations。

postgres=# select usename,datname,count(*) from pg_stat_activity group by usename,datname order by 3 desc;


After that ,  you can make decision where drop a role or not。I also suggest that you shoud check  whether a role owner any objects.