Look at the where the tounge slides, and put something in its way. There is a slot on teh side of the coupler, and you should be able to jam a male garden hose fitting or something in there to keep it from sliding.
Make sure its not the tow vehicle. Send 12 volts to the blue wire and ground the white wire on the trailer. That will unlock the brakes if everything is working right.
Most trailer actuators have a mechanical lockout device for disk. Call the manufacturer of the trailer and they should be able to help. If not you can call me and I will help you find the right solution. I have a few different types in stock but without the type of actuator I wont know what will fit.