You may be able to have a welding shop install some sort of locking screw on the side of the bracket, so that it could be tightened.
Another probably a lot simpler idea would be to get some toothpicks or wooden shims, and try and wedge them between the table leg and the bracket.
But, you also need to check to see if the reason it's wobbly, is the bracket to table leg, or the bracket to floor connection?? It may just be that the screws holding the bracket to the floor have come loose!!
You may want to get some oversized screws and re-fasten it:)