You can do that when logging in - If you just select one vCenter instead of "All vCenters" or "Local Users" - If you don't have that option you need to set it (disabled by default since 6.1).
Under "Administration" -> "Global Settings" -> enable "Allow vCenter users to log in to individual vCenters using the vRealize Operations Manager UI"
The only other option is to create your own dashboard, with a vCenter selector to limit the view... Know there have been feature request for a better handling of this, so if you did one as well it might come up on vmware radar.
Hope it helps