Sure, quarantined items can be deleted. You should probably keep them for a while in case they are "false positive" detections--mistakenly detected as viruses. You can restore such items from quarantine via the ClamWin Quarantine Browser program (Start, All Programs, ClamWin, Quarantine Browser). To verify that quarantined files contain real viruses, you can scan them on the Jotti or Virus Total web sites--just upload and wait for the results). If several of the AV programs (especially a couple of the large AVs like Microsoft, McAfee, Kaspersky, Symantec, or Trend Micro) see an infecton, it probably contains a real virus, and you can safely delete it from ClamWin quarantine. If it looks like a false positive, keep it in quarantine but upload a copy of the file to Clam AV via their web site (click the submit a file menu item) so they can correct the false positive in a few days. Rescan in quarantine after a few days to see if it is still detected and restore from quarantine if it is not then detected.
However, you say that the viruses seem to be returning, which concerns me. The Clam AV scan engine used by ClamWin does a good job of spotting/removing infected files, but it does not do a good job of cleaning up if a virus slips by it. I suggest you download/run the free Malwarebytes cleaner from http://www.malwarebytes.org/
on the web or the free Microsoft Safety scanner at http://www.microsoft.com/security/scanner/en-us/default.aspx
on the web and do a Quick Scan with it. Keep one of them around so you can do an occasional scan for extra security. My personal favorite is the Microsoft Safety Scanner, but you will have to download a new verson once a week or so to get the undated virus signatures. The good thing about it is that you do not have to install it--just run it after you download it and follow the directons. Malwarebytes is more convenient--you can manually download the Malwarebytes signatures at any time after you install it. Malwrebytes can find the most common malware, but Microsoft Safety Scanner can find much more, including hidden malware.