Squeeze with pliers.... only way as far as I know ! Unless you are talking about bloody great big sea hooks you can grind off.
And I got the ideal pair of pliers with completely flat facings to the jaws from a model aircraft hobby shop for less than half the price of the identical items on sale in the fishing shop 100 yards up the road.... ! Worth a look in.
Though do bear in mind that even a squeezed down flat barb can give some "bump" or residual obstruction to removal from the fish and, strictly, it isn't a "barbless" hook, it is a barbed hook that has been de-barbed.
A minute distinction but one that some clubs make, you have to use barbless made as barbless and no pair of pliers or any crushing of barbs etc will do... Check the club rules before losing yr card on such niceties.
Personally, where barb crushing is allowed I like it as the best of both worlds - effectively barbless as far as hook hold in the fish is concerned but that little "bump" irrregularity where the barb has been crushed down helps keep maggots / worms from wriggling off too easily.