If you have a controller on your gear assigned to a slider in renoise with the midi-mapper, and it’s the wrong slider. how do you unsign it again?
AFAK it’s not possible, although it is possible to map the same controller to another slider and the first mapping will be automatically removed.
A remove button would be a nice feature though 
The mapper could use a few more upgrades in general… like discussed in a bunch of other threads.
You could also better ask:
If i mapped a slider to a midi controller and i want to just unmap it how do i do it?
Well, if there is only one assignment, you can do an init upon all the parameters upon the DSP / VST effect (rightclick somewhere to get the popup menu and select “Init parameters”) and the slider mapping will also be removed (All midi mappings will be removed with this function!).
If you have multiple mappings and want to remove just one of them in the same controller, you could map the controller you want to remove temporarily to a slider that has been mapped to a different controller and then the current assignment will move from that slider to the new one (overwriting the previous controller mapping!). (The old mapping from the slider you wanted to clear will also be removed and stated as no mapping). Then you can remap back the previous controller you used on that slider which was mapped to it before you did the swap-action.
It’s a bit of a very much workaround, but this is the only current way i could find (unless the devs can supply some hidden unknown demapping keycombo for this situation)
w00t! Cool, I didn’t know that. Big ups to the almighty Renoise guru, vvoois 
Beware because the sliders are also reset to their default positions, if you don’t want that, then just don’t touch the “init parameter” popup-option.