Who is the "we" you have in mind? The US government has no dog in that fight. But why don't the white farmers get off their butts and hire Blackwater, the private security firm that operates in Iraq? A team like that shouldn't have any difficulty blowing away Mugabe and his low-tech bullyboys. The common people are starving and will welcome the farmers back.
Harry Harrison's 'Stainless Steel Rat' series of sci-fi novels explored this option.
I'm all for private conquest of governments that need *ss-kicking!
Charge the locals a fee, or a royalty on production or exports, and move on to the next 'political re-construction' project.
Hey, it's a dirty job, but somebody's got to do it!
The white farmers won't do it. As much as I love a lot of my friends, a lot of them are racists and rather let Zimbabwe drown if they can't get back Rhodesia and all the white privilege they can carry. That's one reason why you won't see any action on the part of white farmers. Many who would have maybe done something have already left anyway. Another reason, and probably the biggest reason of them all, is that such an action/asassination attempt(which I would endorse)would require widespread cooperation. That's hard to do when the senses are dulled by starvation and communist propaganda.
And just imagine if this was to occur. Zimbabwe would be like Israel, with the exception of Zambia or even Botswana.