HIDDEN SPAM on site

Hello all … I have following problem … someone is posting SPAM links on my site, by editing posts … I checked access log on my apache depending on the date and time of modification of the post and I found following:
220.232.137.251 – – [25/Feb/2008:10:45:23 +0100] “POST /wp-admin/post.php HTTP/1.0” 302 – “https://THIS-IS-MY-DOMAIN/upload.php?style=inline&tab=upload&post_id=-1” “Opera”

The code whch is being pasted at the end of attacked post look like this:

<font style="position: absolute;overflow: hidden;height: 0;width: 0">
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I will welcome any help ….
Thank you
igi