You can manage download availability for your 1-user titles in LibCentral through your Access Permissions Settings.
These settings are located under Settings > Access Permissions. Click on the Permission ID and then the "Owned" tab.

Access Permission Settings for owned titles, showing the preserve online reader options described in following text

There are two settings to limit downloading for 1-user titles:
  1. "Preserve reader access for one user if multiple copies of a 1-user title are owned" - when enabled, if you own multiple 1-user copies of a book, one copy will be preserved for online use only
  2. "Do not allow downloads when only one copy of a 1-user title is owned" - when this is enabled, download is disabled for any 1-user title when only one copy is owned
These settings are not dependent on each other - you may enable both, just one, or neither of these options based on your library's needs. Since the settings are independent, please note that if you only select the first setting, you will only preserve online access for one user if multiple copies are owned, but download will still be possible if only one 1-user copy is owned. If you want to preserve one use for online reader access only for all 1-user titles no matter how many copies are owned, you will need to enable both settings.

Before choosing these options, consider whether you may wish to allow downloads for 1-user titles for a short period (such as 1 day) and disable downloads only for your heavily used 1-user titles. This can be done by using Book Level Permissions for your Owned titles.

You may also preserve online reader access for your 3-user titles: How does the “Preserve 3-user reader access for” setting work?