How much can you prune a hypericum?

As much as you like it turns out. The hypericum has made a comeback.

From being pruned down practically to the ground:

From this:

This is bad, because the intention was to get rid of it. And worse still, it's harbouring snails:

I've cut it back again, but I think part of it lives on the other side of the fence, so I think it will probably keep coming back until we dig it and its roots up. 

On the other hand, if you cut a hebe back, it stays cut back. I've never known one grow new foliage on any stem I've ever cut. With a hebe, pruning is not an option.