p2 repositories don't have a (formal) notion of snapshot and non-snapshot artifacts. So technically you never have snapshot dependencies in a Tycho build, as long as you don't use SNAPSHOT artifacts from Maven repositories via pomDependencies=consider. The latter can be prevented in the same way as in Maven, i.e. by controlling the Maven repositories in your settings.xml (see e.g. this other answer).
But probably this isn't what you are looking for. You probably want to make sure that you don't reference artifacts which will disappear eventually and make your build non-reproducible. For this, you have to check the retention policy of the referenced p2 repositories and make sure that you only reference p2 repositories which are retained "forever". (Example: Retention policy of the Eclipse project p2 repositories.)
If the retention policies are not good enough (or you don't trust the providers to actually stick to them), you need to store copies of the referenced p2 repositories. You can for example download the p2 repositories as zip (or mirror the repository and zip it yourself), deploy it to a Nexus OSS and access it from your build via the Unzip Plugin. (Disclaimer: The Unzip Plugin is an offering of the Tycho project, of which I am a committer.)