What figure are you deleting? Like ashley9803 mentioned, Poser 7 has to store the figure, the textures, and any parented props in memory in case you want to undo the deletion. In some cases, this could take a few moments and it could look like P7 has frozen.
 
Or, are you sure that you've selected the figure? Remember that Poser 7 inherited the "up Library" command from P6: if "focus" is on the Library Palette, hitting the Delete key will move you up the Library instead of deleting an object in the scene.
If there are a lot of items in your Library, it could take a moment for P7 to display the Library and it could look it froze.

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