The best thing I can think of that's cheap is to extend the height of the existing posts using electric sticks (the ones you push in the ground) tied to the existing posts using a couple of plastic pull ties (normally used to hold wires together); then run a strand or two of electric tape (if the electric tape is not readily visible then take plastic grocery bags, cut them into strips and tie them to the strands at fairly close intervals). Hopefully the visible addition would be enough to deter your adventuress. If that doesn't work, I suppose the next step is to actually electrify the addition. Alternatively, if you're going to the trouble of purchasing an electric outfit, perhaps you can create her own little paddock in the current pasture in a location away from the jumping spots - because it's electric and portable it could be moved around periodically.
Hope you can find something that will work, Endiku.