Here is a script that will delete the column along with its default constraint. Replace MYTABLENAME and MYCOLUMNNAME appropriately.
declare @constraint_name sysname, @sql nvarchar(max)select @constraint_name = name from sys.default_constraints where parent_object_id = object_id('MYTABLENAME')AND type = 'D'AND parent_column_id = ( select column_id from sys.columns where object_id = object_id('MYTABLENAME') and name = 'MYCOLUMNNAME' )set @sql = N'alter table MYTABLENAME drop constraint '+ @constraint_nameexec sp_executesql @sqlalter table MYTABLENAME drop column MYCOLUMNNAMEgo