I'm not sure how someone who lives in another country could lock you out of your building, but assuming this is true, if you don't even know what country this guy lives in, you can't get any law enforcement person to do anything and you can't find him to serve him with legal process, so you have to handle this practically.
Cut off his access to you. Obviously at one point you willingly communciated with him so he got your contact information, but you can use the same methods of connecting with him to disconnect. De-friend him on your Facebook account. Change your email address and your school account and/or apply filters to block him, and change your phone number and/or block his calls and texts (and I guess if he somehow got your family's phone numbers, they should change their numbers too or apply the same blocks). If he keeps sending flowers, you can reject those deliveries and instruct the deliveryperson you're not accepting anything from him anymore.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
I concur in all that Ms.Koslyn has offered but I would also suggest that you should consider going even further with respect to minimizing access to you on-line, at least for a year or so. Were I in your situation, I would: close my FB account and all similar social network accounts; cancel my Internet Provider service; abandon all current email accounts and on-line accounts; terminate WiFi in my home; close my cell phone account and rely only on pre-paid disposable phones; and post nothing -- nothing at all -- on the Internet. I would limit my Internet use to "consuming content" only, and only at public terminals such as the public library. Assuming that your description of the problem is all true, and you may be wrong in some of the particulars -- this could be someone who lives down the street from you -- then the person causing your problems is, in my low-tech opinion, a "tech" insider with some highly developed skills and access. Get out of his/her playground all together. I am sure that simply shutting down your on-line persona in all of its manifestations sounds very radical to you and that the un-digital life would be challenging and inconvenient. But, if you are truly in fear and if your fear is based on sound evidence, it may be the best course even with all of the hardship.
I note that your post indicates that you live in Los Angeles. LAPD has a very skilled division of detectives that deal with these kinds of crimes and I recommend that you make a report and seek advice from them.
Ms. Koslyn is very right, in my opinion, in predicting that an attempt to resolve this problem by civil legal action poses a number of problematic issues and is unlikely to be effective.
My responses to questions on Avvo are never intended as legal advice and must not be relied upon as legal advice. I give legal advice only in the course of an attorney-client relationship. Exchange of information through Avvo's Questions forum does not establish an attorney-client relationship with me. That relationship is established only by individual consultation and execution of a written agreement for legal services.