Dictate that all visits to life-bearing planets should be conducted according to maximum quarantine protocols until an extensive biological compatibility survey has been conducted.
Attempt to covertly install monitoring and transmission devices into the industrial civilisation's long-range communications. Make certain that we can monitor the device and self-destruct it prior to contact. Preferably it would self-detect imminent physical contact, detach from the communication lines, and then burn its operating components and denature its structural components, with some sort of filter to prevent overreacting to wild animals...
Washington sounds nice, but try to set up a hidden settlement there. Keep searching the other prospects, perhaps domed colonies could be made with no risk of biological contamination. Also, moons may go through some rather extreme and haphazard seasons, I would like to set up on the unoccupied continent of the earth-like planet.
Build some automated laser-shielded armed solar-powered satellites. Give them a shutdown code and a report code with a limited rate of entry. So we can have them report on their functionality and if people have been feeding them false codes, but that just spamming codes at them won't do anything quickly. Also give them unique variable codes, so that just copying our report signal won't cause all of them to reveal their positions... They should automatically fire on anything new that remotely resembles any of the alien ships.
It would also be nice to work up some sort of targeting assistance, So that weapons will automatically try to hit something if someone points a weapon towards it and pulls the trigger, better still if they could be linked to provide firing fields against evasive targets, but just including a randomiser could largely address that issue. But such a technology doesn't really seem appropriate when we have a civilisation to build and no evidence of enemy infantry.
One we have our colonies established, I should like to hide Liberty with a small on-board colony.