I'm assuming one room with at least 2 WeMos for simplicity's sake... As preparation, I'd have to place wireless cameras at the windows and make sure I can see every angle from my Base Of Evil Operations.
I'd let the lights behave normally for about the first 10 minutes they're turned on with somebody in the room, then make one "flicker" (like an electrical issue might cause) and shut off. Wait for the person to approach the light, turn that WeMo back on, wait for them to head back to wherever they were at, flicker off again when they pass a certain point.
After a couple of times doing that, I'd then start affecting that light plus a second one when they pass close enough to it, and so on with all of the lights in the room. When they get frustrated/upset, turn all the lights back on right after they leave the room, keep them on when they return and sit down... ...well, that is, keep them on just long enough for them to relax, then repeat with some variation, always making sure it always appears to happen in response to something they do or somewhere they go, so it doesn't look random enough to tip them off.
Another version of this for somebody that has a partner currently doing something in another room would be to either just flicker the lamp for short bursts (maybe "WeMo Rocks" in Morse code) *or* do the earlier lamp flicker-die/on/off trick. When the person leaves to tell their partner, wait for the two of them to come, then have it act completely normal, like the original victim was imagining things or something. Wait for the partner to leave, then perhaps make one light at a time flicker and die, or do it to all of them except one -- whatever gets the best reaction.
Damn, if I had enough free time I'd go look through the BOFH website for ideas...