How to exclude dependency in a Maven plugin?

I have a project that needs the following Maven jibx plugin:

  <build>
    <plugins>
      <plugin>
        <groupId>org.jibx</groupId>
        <artifactId>maven-jibx-plugin</artifactId>
        <version>1.2.2</version>
        ...
      </plugin>
    </plugins>
  </build>

Inside the jibx plugin pom, there is a xpp3 dependency which I want to exclude from my project build process (due to some reason I cannot have it inside my private repository).

Is there a way to config my pom.xml (not the plugin pom) to exclude that dependency?

EDIT: I tried to remove the xpp3 dependency from the plugin pom and the project could be built successfully, so I know the dependency is not mandatory.

Here is an example where the jetty-maven-plugin has a dependency on jtidy replaced with a newer version:

http://jira.codehaus.org/browse/JETTY-1339?focusedCommentId=257747&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_257747

    <plugin>
        <groupId>org.mortbay.jetty</groupId>
        <artifactId>jetty-maven-plugin</artifactId>
        <dependencies>
          <dependency>
            <groupId>net.sf.jtidy</groupId>
            <artifactId>jtidy</artifactId>
            <version>r938</version>
          </dependency>
          <dependency>
            <groupId>org.apache.maven.plugin-tools</groupId>
            <artifactId>maven-plugin-tools-api</artifactId>
            <version>2.5.1</version>
            <exclusions>
              <exclusion>
                <groupId>jetty</groupId>
                <artifactId>jetty</artifactId>
              </exclusion>
            </exclusions>
          </dependency>
        </dependencies>
[...]
      </plugin>