I brush mine away with my wife's stiff washing up brush. ( Hope she don't read this or she'll know where it went!)
I paint the trunks with white arboricole I make up from quick lime and milk and it's impirtant to get the trees cleaned of lichen before application.
On one or two of the older trees I've found problems developing under the lichen which i might not have discovered early without cleaning off the parasite lichen.