The Collaborative International Dictionary of English v.0.48:

Dull \Dull\, v. t. [imp. & p. p. Duller; p. pr. & vb. n.
   Dulling.]
   1. To deprive of sharpness of edge or point. "This . . .
      dulled their swords." --Bacon.
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            Borrowing dulls the edge of husbandry. --Shak.
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   2. To make dull, stupid, or sluggish; to stupefy, as the
      senses, the feelings, the perceptions, and the like.
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            Those [drugs] she has
            Will stupefy and dull the sense a while. --Shak.
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            Use and custom have so dulled our eyes. --Trench.
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   3. To render dim or obscure; to sully; to tarnish. "Dulls the
      mirror." --Bacon.
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   4. To deprive of liveliness or activity; to render heavy; to
      make inert; to depress; to weary; to sadden.
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            Attention of mind . . . wasted or dulled through
            continuance.                          --Hooker.
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Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "dulling":
   abating, allaying, alleviating, alleviative, analgesic, anesthetic,
   anodyne, assuaging, assuasive, balmy, balsamic, benumbing,
   blunting, cathartic, chastening, cleansing, cushioning, dampening,
   damping, deadening, demulcent, diminishing, easing, emollient,
   lenitive, lessening, mitigating, mitigative, narcotic, numbing,
   pain-killing, palliative, purgative, reducing, relaxing, relieving,
   remedial, softening, soothing, stunning, stupefying, subduing,
   tempering