Cut the ivy off at the base of the trees - you might want to poison the roots of the ivy if they are very large and can't be dug out. Do this using SBK, drilling holes into the roots (not right through, just about quarter inch deep) and then filling the holes with the brushwood killer. Cover and leave, but don't, whatever you do, spill the SBK -its very watery and you have to be careful.Any ivy left on the tree will die back.