Well we do have laws against openly bribing lawmakers.

Lobbyists aren't handing over wads of cash, they are spending money on things like ad campaigns, surveys, studies, talks, busing people in to talk to their representative, etc which are meant to legitimately sway the opinions of both the elected officials and the electorate. Maybe to get more politicians to show up the lobbyists pay to serve nice steak dinners at a talk, and in the extreme maybe the talk is just a thinly veiled guise for what is really an expensive gift of steak dinner, but conceptually it's no different from any other potential catering option, and there is no simple way to ban it. Sure you can make specific schemes illegal, but there will always be another loophole. So long as you allow people to interact with their elected representatives and speak freely, there will be people who have their politician's ear. You might be able to eliminate lobbying by sequestering the politicians throughout their term, but that's going to have the opposite of the desired effect of making government more democratic.