Don't want passcode to access voicemail

The passcode is with your cellular carrier; you would need to contact them but I doubt they have an option to have no passcodethen as it would be possible for anyone from any phone (cellular or dial-up) to access your voicemail through the carrier if you had no passcode. Once you have entered it one time on your iPhone you shouldn't need to enter it again -- which is why many, many people forget their voicemail password of the fact that they even have one.

You can't turn off voicemail in general due to the fact that it's connected to the cellular company. By default, because Cellular Data is turned on, Visual Voicemail is enabled on every iPhone which lets you select a voicemail in a list and listen to it rather than listening to the automated system and listening to voicemails the "old fashioned" way. If you turn off the Cellular Data option (Settings > Cellular > Cellular Data slider), then the Visual Voicemail will be disabled and the password won't be asked for until you call the voicemail number. Visual Voicemail is a process in the background that checks for voicemails periodically and when it tries to look for a voicemail it can't because it doesn't have your voicemail password. You either need to find the code, change it, or contact your cellular carrier to get that issue resolved. I would always make sure your password is secure (as like every password) so no unauthorized users can get in.