How can I turn off the PoE on Eth1?
I am using Eht1 for WAN and when I plug into a switch that has PoE, the RB2011 goes down.
Workarounds I have thought of: 1. cut Ethernet centre wires (blue pair). 2. Reprogram RB2011 to use Eth2 for WAN

What does it mean has PoE ? Device 2011UIAs has:

a) PoE in on ether1
b) PoE out on ether10

What's wrong on having wan on e.g. eth7 ?

It is unlikely that the RB2011 passive PoE would be affected by "a switch that has PoE" (normally 802.3AF/AT).
There is probably some other incompatability between your switch and the RB2011, e.g. something related to spanning tree.

If you connect a RB2011 with eth1 to a PoE 802.3af/at enabled port, the RB2011 will go down. This is mostly because of overvoltage protection.
Tested on several Standard-PoE-Switches.

That is strange, a 802.3AF/AT switch should not put 48v on a device that is not prepared to be powered using that standard.
But anyway, you can just turn off the PoE on the involved port and see if that fixes it.

There might be an alternative explanation:
Lots of switches apply negative power (eg. -48VDC)
This means the voltage applied to the RB2011 is reversed. Thus it switches off.