You can do it with launchd, if you must. There are two ways:
Code:
<key>ProgramArguments</key>
<array>
    <string>/bin/rm</string>
    <string>-f</string>
    <string>~/Library/Caches/com.apple.Sarari/Webpage Previews/*</string>
</array>
<key>EnableGlobbing</key>
<true/>
or
Code:
<key>ProgramArguments</key>
<array>
    <string>/bin/bash</string>
    <string>-c</string>
    <string>rm -f ~/Library/Caches/com.apple.Safari/Webpage\ Previews/*</string>
</array>

Note that in the first form the space in "Webpage Previews" should not be escaped, since the whole point of specifying ProgramArguments as an array is to describe the end result after shell expansion, so that word splitting and quote removal have already been done. (EnableGlobbing has the side effect of forcing another round of quote removal, so escaping the space will do no harm, but you should not get in the habit of doing unnecessary quoting. In the absence of EnableGlobbing, any quoting characters will be passed along to the process being launched.) In the second form, on the other hand, we're introducing another copy of the shell, which will do all the normal command-line expansion, so normal command-line quoting rules apply.

But perhaps easier would be eschew launchd and use Folder Actions instead:
Code:
on adding folder items to this_folder after receiving added_items
    repeat with afile in added_items
        tell application "System Events"
            delete afile
        end tell
    end repeat
end adding folder items to

Note that you tell System Events to delete the files. If you tell Finder to delete them, it'll just move them to the Trash, cluttering up your Trash. (I hate it when programs do that. It's my Trash, for files I delete.)