Display only

We wanted to use a Mac Mini as a display only.

That means: jaut a display, no keyboard, no mouse.

Just display a web page.

No problem? Try it. The Mac Mini wants a mouse. It tells you so. Every time you boot it.
Every time the mac starts up, a dialogue appears telling you that you're an idiot because you did not connect a mouse.

OS X in that case tries to set up a mouse using using bluetooth.

So, one way to eliminate this dialogue would obviously to remove the app.
It's this one:

/System/Library/CoreServices/Bluetooth Setup Assistant.app

But that's not a very elegant way to do this.
Besides there is always the risk of an update putting the app back in place thus the dialog appearing again.

After having looked at the app I found a reference to 'com.apple.bluetoothsetupassistant'. I googled a bit I and found a solution on this page:

Managing Bluetooth, or pesky Apple Preferences « Managing OS X

There it says:

# turn off the Bluetooth KB/mouse dialog that comes up if no KB/mouse is found
`/usr/bin/defaults write /var/root/Library/Preferences/blued BluetoothAutoSeekHIDDevices -int 0`;

# restart the Bluetooth daemon
`/usr/bin/killall -HUP blued`;

Or you might open the Bluetooth Preference Pane and uncheck “Open Bluetooth Setup Assistant at startup when no input device is present”

That will do it as well, just in case ;-)