Temporarily change the Comments field setting to not log the edits. Open all the records in Grid Edit and highlight that column and delete the data. Then, go back to the comments field and turn the log back on.
There is a way to make a formula URL button to do this in one click. It would take some setup and testing, so not a formula I ca; post, but could be done with an hour if Setup. Assuming to intent is to clear a field set to log edits.
FWIW, using script you can turn a text multiple line field into a custom logging field that retains only the last n edits and you can gain complete control over the information and format that is being logged.