Dell doesn't post how to "enable" amt firmware or bios features. This is done at the factory.
If you disable AMT you must replace the motherboard because this can only be done at the factory at the time of purchase.
Am I allowed to post external links here? I took screenshots and documented how to do it, I just don't know where to post it.
It's really not that difficult. You copy the data from flash, modify a flag, and reflash your motherboard. Literally takes less than five minutes.
I'm working on organizing my screenshots into an imgur album. I did this on three different Optiplex 7040's, so I'm confident it can be done. Keep in mind that you are modifying your system flash and doing this could render your system unbootable if there is an error. I'm not *recommending* that you do this, I'm just showing that it is possible.
Album (completely a mess) here: https://imgur.com/a/Mh9Jz
Essentially, you need to close a jumper on the motherboard marked "service mode." I used the PW reset jumper since I didn't have a spare laying around. You'll need to get a copy of Intel's Flash Image Tool kit - you're on your own, there - and using those tools, you can see the commands I ran to get the flash into a file, modify the AMT flag, and reflash the resulting file in the screenshots.
I really want to stress that this is firmware level modification that, done incorrectly, could render your system unbootable and require a hardware replacement. Only do this if you are willing to tinker and don't care about any sort of warranty support from Dell. Your best bet is to make sure you get a machine with AMT enabled from the factory. While *I* did it, I don't know you, or your technical skillset. That being said, I'm not a super genius - I figured this out from lots of google searching.