If you only want to clear out the bugs you've submitted and keep your users and other customizations, you can issue the following SQL to the database
[here I am assuming you're using mysql, your mantis database is named 'bugs' and the table prefix is 'mantis_', the default:
use database bugs;
delete from mantis_bug_table where 1=1;
then, if you want your bug numbers to start once more from '1',
issue this SQL statement:
there are other tables you'd want to truncate as well to clear out bug data:
this SQL should do it:
-- remove all bug data, text, and history
DELETE FROM `mantis_bug_table` WHERE 1;
DELETE FROM `mantis_bug_history_table` WHERE 1;
DELETE FROM `mantis_bug_relationship_table` WHERE 1;
DELETE FROM `mantis_bug_text_table` WHERE 1;
DELETE FROM `mantis_bugnote_table` WHERE 1;
DELETE FROM `mantis_bugnote_text_table` WHERE 1;
-- re-set the autoincrement fields for those tables so that new
-- defects start at '1'
ALTER TABLE `mantis_bug_table`AUTO_INCREMENT = 1;
ALTER TABLE `mantis_bug_history_table`AUTO_INCREMENT = 1;
ALTER TABLE `mantis_bug_relationship_table`AUTO_INCREMENT = 1;
ALTER TABLE `mantis_bug_text_table`AUTO_INCREMENT = 1;
ALTER TABLE `mantis_bugnote_table`AUTO_INCREMENT = 1;
ALTER TABLE `mantis_bugnote_text_table`AUTO_INCREMENT = 1;
There's only one problem, if you look up using this forum search engine nothing like this comes up, but if you use GOOGLE search with simple key words "mantis+clear+database" then you have a lot of previous information.