deleting 500 posts of spam user

[ketanco]

When i try to delete the posts of a spam user if it is more than 50 it tells me to go to control panel

i dont want to do it as it takes time, so how and where can i modify this 50 number to say 500? support told me to come to this forum to do that

FYI note: when i asked this to support, they told me to go to control panel first but then they said there is a bug and it can not be done in control panel and i must go to user about page and delete all his posts from front end 25 each time max, which is very time consuming if there is a lovely spam bot which posted 500 posts