Remove ramp from z+1 layer

EDIT See my comment below, this question is solved. HOWEVER, please do read, as the reason I use this (cart-only entrance), doesn't seem to work. That is, I have the construction as defined in the image below (except for a ramp on z-1, x2, y0, with a floor above it), but using D to show trader routes, still shows the path as inaccessible. Any thoughts on this?

Can anyone explain to me how I can create the following picture (as shown on the DF Wiki):

z = -1  z = 0
▓▓▓▓▓  ▓+++▓ 
▓+++▓  ▓+++▓  
▓▲▲▲▓  ▓▼▼▼▓  
▓+++▓  ▓+++▓  
▓▓▓▓▓  ▓+++▓  

I am having troubles removing all but the middle ramps in the Z -1 layer, without getting any dwarfs trapped.

What I currently have:

z = -1  z = 0
▓▓▓▓▓  ▓+++▓ 
▓+▲+▓  ▓+▼+▓  
▓▲▲▲▓  ▓▼▼▼▓  
▓+++▓  ▓+++▓  
▓▓▓▓▓  ▓+++▓  

What I tried (0,0 = top left):

• build floor at z0, x2, y1, remove floor, ramp still there
• Channel z-1, x2, y1, build floor at z0, x2, y0+1, remove floor at z0, x2, y0, floor at z0, x2, y1 now collapses on ramp, but ramp still there.

I'd like to do it within the confines of the above axises, and I can't use z1 due to being close to other caves.

The ramp is natural (well, by Channeling), not by building a ramp manually.