welcome to GoY.
generally they can be pruned quite hard.
this is the general pruning advice:

Prune hypericum in early spring. Inspect the stems for signs of winter damage and cut off any that you find. Cut them back to a main branch. Prune off shoots that have bloomed, as they will not bloom again. Cut these back to where they meet a main branch. Cut the entire shrub to within 4 to 6 inches of the ground if it has got totally out of hand.