I unflagged the member, and the tasks were canceled.
From now on, I think I'll manually delete posts before deleting a user account or removing a spammer posts, as the task system doesn't seem to be working anymore.
UPDATE `forums_forums` `forums_forums` SET `topics`=2329,`posts`=-186 WHERE id=14
Out of range value for column 'posts' at row 1
Backtrace
#0 /home//htdocs/site/system/Db/Db.php(1159): IPS\_Db->preparedQuery()
#1 /home//htdocs/site/system/Patterns/ActiveRecord.php(520): IPS\_Db->update()
#2 /home//htdocs/site/system/Node/Model.php(2575): IPS\Patterns\_ActiveRecord->save()
#3 /home//htdocs/site/system/Node/Model.php(1086): IPS\Node\_Model->save()
#4 /home//htdocs/site/system/Content/Item.php(2843): IPS\Node\_Model->setLastComment()
#5 /home//htdocs/site/applications/forums/sources/Topic/Topic.php(1116): IPS\Content\_Item->delete()
#6 /home//htdocs/site/init.php(947) : eval()'d code(942): IPS\forums\_Topic->delete()
#7 /home//htdocs/site/applications/core/extensions/core/Queue/MemberContent.php(126): IPS\forums\tthumb_hook_forums_Topic->delete()
#8 /home//htdocs/site/system/Task/Task.php(55): IPS\core\extensions\core\Queue\_MemberContent->run()
#9 /home//htdocs/site/applications/core/modules/admin/system/background.php(87): IPS\_Task::runQueue()
#10 /home//htdocs/site/system/Helpers/MultipleRedirect/MultipleRedirect.php(93): IPS\core\modules\admin\system\_background->IPS\core\modules\admin\system\{closure}()
#11 /home//htdocs/site/applications/core/modules/admin/system/background.php(138): IPS\Helpers\_MultipleRedirect->__construct()
#12 /home//htdocs/site/system/Dispatcher/Controller.php(107): IPS\core\modules\admin\system\_background->process()
#13 /home//htdocs/site/applications/core/modules/admin/system/background.php(42): IPS\Dispatcher\_Controller->execute()
#14 /home//htdocs/site/system/Dispatcher/Dispatcher.php(153): IPS\core\modules\admin\system\_background->execute()
#15 /home//htdocs/site/admin/index.php(13): IPS\_Dispatcher->run()
#16 {main}