You can prevent ClamXav from scanning certain files or folders by listing them in this Exclude Files preference section.
The easiest way to do this is to drag and drop into the preferences panel the item you want to exclude.
Items can be enabled/disabled on an individual basis via the checkbox in the "Enabled" column. You can ignore the "RegEx" column.
Note for power users
As well as dragging and dropping files directly into the preferences panel, ClamXav also allows you to specify text patterns to determine which files should be excluded from a scan. These patterns can either be plain text or can be a complex regular expression. If you are wanting to use a regular expression, you must select the appropriate checkbox in the RegEx column.
The "Comments" column lets you enter a description to help you remember why you're excluding a file or folder, or what the pattern does — the comment has no impact on the files being scanned.