Psalm 91 is a traditional prayer for divine protection: indeed, in folk magic traditions it is recommended as a way of seeing off evil influences such as (alleged) black magic. So I would suggest praying this Psalm whilst feeling gratitude to God that He will send His Angels to ward you from all evil influences.
Otherwise there are other protective rituals in existence, such as the good old Lesser Banishing Ritual of the Pentagram (google it if you’re not sure what this is).
Sol Ascendans - The Website of Alex Sumner 