A
About the only way to stop the update from happening again is to root the older version you flash. If you do that, you might as well install TWRP as well. Then way, in the future, you can have your data backed up and if you have to flash another ROM you can restore your data easily.
Can I delete the new update on my S8
Not easily. The new update replaced the old version of the OS. Even a "factory" reset will not remove the new version. You would need to back up your data, find the file to flash, use ODIN to flash that, then set the phone up again from blank and restore your information. Once all that's done, you get to tell the update notification to "shove off" until it just decides you've delayed enough and installs on its own, leaving you in the same situation.
Maybe a better option - what don't you like? Maybe we can help guide you to resolving the issues.
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I asked the same yesterday - is it possible to downgrade easily. If it isn't, then unless there's some really compelling reason to install an update, I don't think it's worth the risks involved. Software testing certainly isn't what it once was. I speak as a software developer.
I can confirm that enabling the developer options and then turning off Auto Updates stops the phone nagging about the update (throwing up a modal dialog box that fills the screen where you might accidentally click the button to proceed with the update).
My phone had already detected the update so it's stuck with a little "1" over the top of the settings icon indicating there is an update available but it doesn't keep trying to railroad you into accepting it.
Bit late if you've already upgraded but might be helpful to others who don't want to risk an update, and I can't honestly see anything in the "Oreo" update which is of any practical use to me though that's not to say it isn't useful for others.