You should be able to adjust the spring preload - and I would do it all the way up. I believe that on the Supre 8 150 it is at the bottom of the shocks. There should be a ring that the springs set on, and it has notches (ramps) under it that sit on a pin. You should have a spanner wrench in your tooklit that fits on there and allows you to turn that ring, and increase the preload. Move it to the notch that is farthest from the spring - but don't go past it. If you happen to have shocks that don't have that ring and notch setup (usually 5 notches) - then no, you can't adjust them...
If you are already on the farthest notch or if you have no notches - then you either need to buy better shocks - or take those apart and add spacers. Taking a shock apart to add spacers is not as easy as it may sound. You need to compress the spring and then remove the collar. It's best to take the shock to someone who has a press...