Unfortunately it is not possible out-of-the-box: Merge activities are recognizable by modification of svn:mergeinfo property.
The problem is, you can not see if a change was made in content of a file/directory or if change was made in some properties by looking into the svn log. You need to make a diff between these revisions.
A very easy workaround could be that useres performing a merge operation should add a keyword to log message(for example:MERGE). It is quite easy to check via pre-commit hook if the keyword is present in a certain log message