There are two sets of logs, one of which you can delete individual entries from & the other you can't.
When you first enter the "Log" application, you have "Recent calls", "Call duration" and "Packet data". Entering these will allow you to delete individual entries (as the manual describes).
However, the comprehensive list you get by pressing the right-key will only allow deletion of ALL entries. This will also clear all entries in the other lists.
The only way around it is, as you say, deleting all entries. Or setting the log duration to a shorter interval (1 day, for example).
You need to ask yourself: how important is a log at all?? Especially if one or two inappropriate entries are in the list!