The button in your screen shot is a button on a DockManager pane. It's not part of the ExplorerBar. The ExplorerBar doesn't have a close button. 

To disable the close button, you would do something like this: 

var pane = this.ultraDockManager1.PaneFromControl(this.ultraExplorerBar1);
pane.Settings.AllowClose = Infragistics.Win.DefaultableBoolean.False;

Add on information,

the disable closed menu was setup on ultradockmanager1 with set false on ShowCloseButton.

but it will helpfull if there is a function available how to call back the closed explorerbar menus.

I did disable close menu last time due to i have no idea to call it back again when user did close it.

I'm still not sure what button you are referring to. The ExplorerBar has a number of different styles. The only thing I could find that even resembles a menu that closes are the flyouts in the OutlookNavigationPane style. But I don't think there's any way to prevent that from closing since it's essentially a context menu. 

Maybe you can post a screen shot of what you are referring to? 

Hi Mike,

it's about the closed (x) menu on ultraexplorerbar side menu,  last time i stuck how to prevent user close it. () 

i have solved my problem on my last project time with do some set value action on property on ultraexplorer bar (cannot remember which part i did), so user will not able to close menu, only able to change state hide or auto hide.

if it has some guide note about it, it will be helpfull for us,