I saw a post on XDA about this. You can't disable LTE, but what you can do, is add an APN specifically for HSPA+ and select it.
Let me see if I can find the post.
Go to Settings, More Settings, scroll down to Mobile Networks, click Access Point Names, press Menu and click New APN and then add this,
Quote:
Name: AT&T WAP
APN: wap.cingular
Proxy: <not set>
Port: <not set>
Username:
[Only registered and activated users can see links. Click Here To Register...]
Password:<not set>
Server:<not set>
MMSC:
[Only registered and activated users can see links. Click Here To Register...]
MMS proxy: wireless.cingular.com
MMS port: 80
MCC:310
MNC:410
Authentication type: <not set>
APN type: default
APN protocol IPv4
then go back in and make sure this is the selected APN and it should drop you from LTE and only connect to HSPA+