Two good methods:
1) Use an
env_tonemap_controller. Use a logic_auto to fire off some outputs, specifically "SetBloomScale blah". You can also use SetAutoExposureMin and SetAutoExposureMax to set the exposure range. Throw the entity in your map and use ent_fire to play with it and find some good results. This way is obviously global, so all light sources will be affected.
2) Use the BrightnessHDR or BrightnessScaleHDR keyvalues to tweak the HDR on the individual light source, so no other lights get scaled down. You should be able to use decimal figures to scale downwards.