yes u can delete the default constraint. First find out the 
constraint name for the default and then execute the 
following quey

alter table <table-name> drop constraint <constraint-name>

table create time must constraint name is important..create
time not use constraint name don't delete don't delete
constraint column 

alter table table_name drop constraint <constraint name>
table create time use this query 
create table delete_table(emp_id int constraint del_pk
primary key,age int)

constraint name delere query
.............................

alter table delete_table drop constraint del_pk

As i understand this question is not about removing the 
constraint in a column. It is about removing the columns 
containing default constraint. 
If it is about removing the default constraint then given 
answer no. 1 is absolutely correct.It's quite a bit similar.
 However, if you want to remove column containing default 
constraint,first you need to drop the default constraint 
then you can remove the column/'s :- 

First remove the constraint :
eg :- as answer 1. 
step 1:- alter table <table-name> drop constraint 
<constraint-name>

step 2: alter table <table-name>
drop column <column-name>