How can I disable the event sounds in Kubuntu?

Just go to Application and System Notifications in System Settings and select Player Settings tabs marking No audio output:

KDE Userbase: http://userbase.kde.org/System_Settings/Application_and_System_Notifications

Manage Notifications

Some applications and subsystems of your workspace can notify you about various events. Here you can specify which events you wish to be notified about, and how you want to be notified.

System Settings > Application and System Notifications > Manage Notifications.

Pick the application. The KDE Workspace has the trash, logout, login etc notifications.