The type of air lock you are using has a taper at the bottom, with an "x" cross hatch cut into it. You can cut the taper off to create a larger diameter tube for the krausen to flow through.
Do you have a siphon hose that will fit over the vertical part of air lock, under the floating cover? Slip your hose over this and put the other end in a bucket to carry the blow off away. Have some sanitizer solution in the bucket to prevent fruit flies, bacteria, etc. from entering the hose.
A blow off tube can also be fit into the air lock bung or into the lid by removing the air lock grommet. Just depends on what outside diameter hoses you have.
If all else fails remove the air lock and set an inverted glass bowl over the vent hole. Let the krausen flow out until fermentation slows, and the air lock can be replaced. Good to have the bucket in a tub to contain over flow.
Is your fermentation over active because of uncontrolled fermentation temperature? Is your bucket to small for the volume of the recipe, eliminating head space for the krausen?
edit: Sanitize anything used.