Your hulls aren't flexing - the trampoline frame is just loose. Unless you're racing, it's not a big deal, but it will get worse.
First thing to do is lace the tramp as tight as you can get it. The sidebars will bow inward if you get it tight enough.
If that doesn't tighten the frame up enough to your liking, you can shim the pylons where they go into the castings. Hobie makes a kit with brass strips, or you can use aluminum strips cut from soda cans. I've even used strips of vinyl siding.
If that doesn't work, you can glue the frame together. That's kind of the ulitimate stiffening method. It does make it hard to get the boat apart if something breaks.
You can seach this forum for ways to do any and all of the methods listed above.