How can I disable multitasking in iOS4?
You may be worrying more than you need to be.  Apps can only use multitasking for certain functions:


Playing audio
Tracking your location
VOIP
Local notifications and alerts
Complete a lengthy operation


In all cases, rather than the app remaining running, the system keeps it suspended and awakens it at appropriate times, such as when it needs to fetch more audio.  If the app uses too much CPU while it's in the background, it gets terminated.  If the foreground app needs more memory, then the background apps will get terminated as necessary.

The apps in the multitasking menu aren't necessarily consuming any resources or battery life, it's more like a "most recently used" list.  Apps will still be listed there whether or not they are actually running.  You can help prove this to yourself by turning the phone off and on; the recent list will still be populated even though obviously all the apps are terminated.