How can I fail the build when code coverage is too low?

Q: Can I fail the build - or block a deployment - when code coverage drops below a given percentage?

Yes. Set the targetPercentage parameter and call the clover:check goal (Maven) or the <clover-check> task (Ant).

<configuration>
  <targetPercentage>75%</targetPercentage>
</configuration>

mvn clean clover:setup test clover:aggregate clover:clover clover:check

If you want the build to succeed anyway and only print a warning, use -DfailOnViolation=false.

See Configuring a coverage goal for Maven and <clover-check> for Ant, including how to ratchet coverage up by failing the build when coverage drops relative to a previous history point.

Blocking a deployment on a CI server

There is usually no separate "coverage gate" setting on a CI server - this is Clover's configuration, not the CI server's. Because clover:check fails the build when the threshold is not met, simply make your deployment depend on a successful build.