My are contained with the Invisible Fence system. We have a problem with keeping a neighbor dog out of our yard. What do you suggest?

I have been reading the question and answer section as long as I can tolerate sitting here. I am sorry to bother you, I recently had back surgery and cant sit here any longer.

I have 2 dogs, 5 year old mushball of a Boxer and a young 1 year old American Bulldogg. I keep my dogs sleeping in a crate at night. During the day they are out on our 2 acres to run freely. They are contained with the Invisible Fence system. They are very people friendly. Problem is keeping a neighbor dog out of our yard....and the attacks are getting worse and more severe....so far there have been 3 instances. I am afraid that my American Bulldogg could kill this dog if it keeps coming around. The visiting dog is a Pit /Shar Pei cross, I have witnessed her enter the yard, tail up high over her back head help very high, she approaches very abruptly, within seconds.... FIGHT. Very scary my children have been within feet of these dog fights and they do not understand to get the heck away. I am at a loss as to who is responsible here? I am also very saddened
by the fact that my AB has now drawn blood.

Thank you,
Dawn
Wisconsin