WordPress spam comment is very disturbing and can take so large size of database. From my experience for around 18.000 spam comments we will get 19 MB of spam database. Deleting via wordpress dashboard (in my case) didn’t work, si i have to manually delete this spam comments via PHPmyadmin.

to delete spam commment
1. go to PHPmyAdmin
2. choose your wordpress database
3. choose dbprefix_comments table
4. choose sql in menu bar
5. enter sql command
DELETE FROM `dbprefix_comments` WHERE `comment_approved`=’spam’
6. Choose ‘Go’ or other button than ask you to run the command
7. viola, your spam comments is now deleted 😀