Is it called a sofit? (I'm not English) The piece that closes the space between top of cabinets and ceiling I mean. It can be as simple or as elaborate as you like. If you go for a simple style, I would not make it flush with the cabinets, just recessed a little gives a better look I find.

(hover over the pics to see the sofits)

I'm definitely looking for cost effective, forgot to mention that. Thank you!

Is that an exterior wall? If so, you could cut through the back of the cabinet, reroute the vent, and then patch the drywall where the former vent was. Extending the cabinet with a flat stock board and adding crown is another solution, but matching the cabinet stain and wood could be tricky. You would also want to use a similar technique on all the upper to tie it in. Cost still wouldn't be terrible if you DIY.

Buy a sheet of finished wood from the cabinet manufacturer to cover a new soffit above just that cabinet. The piece should be available if the cabinets are fairly new. Alternatively, just build a small soffit above the stove cabinet and paint to match the walls.

I agree with the crown molding suggestion. You could also stack the crown molding by putting a flat trim piece and then a fancy coved crown on top of that one.

Paint it "out" or (even though I hate fake greenery) a basket of fake greenery that flows front and sides of it is cheap too.

Adding a thick moulding would not only help in hiding this but would also enhance the look of your cabinetry. I would box in and paint around to further mask it.

A soffit just above that cabinet would be fairly cheap and easy to do...
I'm thinking a U-shaped wooden frame (on its side), covered with mdf or plywood or whathaveyou, painted in the colour of the wall.

If that is standard duct tape, I believe it is the wrong kind. There is a shiny aluminum (not fabric) tape that I believe is the kind you want there. After fixing that, if you're not going to get more of the materials from your cabinet maker and add mouldings on top of all of your cabinetry, you might consider placing a section of rigid (not accordian-style) duct over it. You could use metal primer and then paint it the color of your walls. Good luck!

You can glue fake flowers with hot glue gun so that you can take them of. Like these on the picture.

Perhaps just simply box it in - 3 sides - slide it on. Paint it the wall color and ignore it.