Number when using "one of those who..."

prefer refers to those guys, and should be in the plural - Those guys prefer.

Those guys prefer studying to travelling. He is one of them. He is one of those guys who prefer studying to travelling.

You confused yourself by the way you phrased the question. The verb "prefer" is not "after" a "clause", it is part of the object of a prepositional phrase. The main verb of the sentence is "is", rendered properly as singular. And "those who prefer studying over travelling" is a phrase that acts as a plural noun. So Centaurus is absolutely right; the verb "prefer" must be plural.