[maven] Is there a simple way to remove unused dependencies from a maven pom.xml?

As others have said, you can use the dependency:analyze goal to find which dependencies are used and declared, used and undeclared, or unused and declared. You may also find dependency:analyze-dep-mgt useful to look for mismatches in your dependencyManagement section.

You can simply remove unwanted direct dependencies from your POM, but if they are introduced by third-party jars, you can use the <exclusions> tags in a dependency to exclude the third-party jars (see the section titled Dependency Exclusions for details and some discussion). Here is an example excluding commons-logging from the Spring dependency:

<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring</artifactId>
  <version>2.5.5</version>
  <exclusions>
    <exclusion>
      <groupId>commons-logging</groupId>
      <artifactId>commons-logging</artifactId>
    </exclusion>
  </exclusions> 
</dependency>