One by one unfortunately.
I load up 5 - 6 tabs, delete them
load another 5 - 6 etc etc
it takes a VERY long time.
We have asked WGT for this for a very long time.
You can select multiple friends to challenge them to beat your score, should be the same to delete or sent multiple messages all at once.
Imagine the time we would save if we could do this, time better spent playing golf = using up balls - buying more = spending more credits...