no big difference, mainly denpends on speaker's usage

防止: prevent (not happened) 阻止:stop (happened)

阻止 a thing have happend ,we do something to stop it. 防止 do something to avoid it happen.

防止：prevent something that maybe happen in the future. e.g. 我们要防止此类事情再次发生。 阻止：stop something is happening or may happen in the future. e.g.我们要阻止他们继续杀人。 Besides，防止 is more formal than 阻止.

防止 means"prevent,avert,hinder,guard against,protect against";阻止 means"to prevent from, to keep from, to prohibit from, to stop from, to hold back".