The latest Lansweeper releases don't just identify anti-virus software based on a comparison between scanned software and the anti-virus list. They also pull anti-virus software from WMI, specifically from the AntiVirusProduct WMI class. The Antivirus section of your computer's Software tab shows anti-virus software pulled from WMI, which is why the machine is deemed to have anti-virus software installed. Modifying the anti-virus list will have no impact on the machine's anti-virus status, as the anti-virus item listed on the computer page is pulled from WMI.
The first thing you should do is rescan the machine by clicking the Assets link at the top of the web console, ticking the checkbox in front of the asset and hitting the Rescan button on the left. If the anti-virus software is still listed after rescanning, it is still stored in WMI and what Lansweeper is reporting is technically "correct". WMI is saying that there's anti-virus software. A "solution" to this problem would be to exclude anti-virus software detected through WMI from your report, i.e. by removing this from the query:
And tblAssets.AssetID Not In (Select
tblAntivirus.AssetID
From tblAntivirus)