Use a max repel to cancel the random wild things encounters. Reset the radar if you run into something you don't want; I'm not sure that it really does increase the rate of encountering common pokémon--I think that might be an issue of perception rather than function.
Anyway, I used to think the pokéradar was a worthless item. Then I learned about shiny hunting. I may have no patience whatsoever and consequently not be able to do it, but some people have gone nuts with it, and it's a cool idea.