Most of us are familiar with this issue and it happens to a lot of people on servers *Mostly Popular ones or ones with less rules, mostly happens to females not so much to males that I have seen* and it can get very annoying if this happens to you a lot on servers you play on where as to where you can't go five minutes without someone hitting on you throwing stuff at you or generally asking you out or flirting with you *Mostly happens on servers not run or watched over by many moderators,popular servers or a server with a generally bad community* And it can sometimes in most serious cases drive you to not playing, or not often playing on your favorite servers because of how annoying it can be, But there are reasons this can happen,
1.Your Skin
*Sounds a lot like a real life thing* But it can More often than not be the skin you are wearing that drives people to you, They Might think its cute/adorble or *Hot* looking.
2.Your Username
It could also be your username that drives people to want to flirt with you etc, Even if you don't have a skin they look at your username and think it sweet or adorble,etc.
Those are mostly most of the reasons people flirt with you out of the blue and why people on servers will flirt or give you things or ask you out on servers or ask you for your skype*Dude I just met you less than a second ago why do you want my skype you don't even know me Situlation*
3.Your Personality
If you go and chat on servers a lot this could also be a reason people would do or ask you the following above maybe they've seen you in chat a lot and think you awesome*Though mostly not the case if you don't  hardly or not at all talk on some servers*

These things can be annoying and can be hard to pervent unless you never go on another server again or go on servers that have these types of people on them.
4.Rarity of Females On servers*
I haven't seen this happen to guys that often actually I haven't seen it at all but this fourth part mostly applies to females though can to apply to males on servers with a lesser count of them but yeah believe it or not not a lot of girls play minecraft if you put it to a ratio boys to girls, and this is mostly proven by how very few or not a whole lot of  females are found on some servers *Examples MiniGame servers,Survival servers,Facations servers,PVP servers,Just about any server really, And some dudes in minecraft can be desprate to find one and once they do they go in for the kill*I have no idea why they would I mean its minecraft I would rather have a real life partern than a game one but eh*

But there are ways to prevent these things from happening if you still enjoy going on servers that may have people like this.
1.Wearing a Animal skin,Gender Netural,Robot etc
Bascially any skin that doesn't have a specific  gender by a simple glance or a mask over your face *1.8 Useful for taking on and off*
2.Derping your skin up/changing your facial look
Usually your facial look on your skin is the likely reason that people may like about your skin that makes them want to flirt with you in minecraft *Example The cute look of cute skins/Innocent example:*Basically anything that doesnt look cute/innocent * there are some ways you can change some facial features to make people less likely to flirt with you by simplely changing your facial features Examples:Derp Face: Serious Look:Netural Expression: Chill/Serious/Okay Not too Innocent Looking but not too litte: Too Kawaii/Odd Looking: *Yes I used my Own skin as a example*Note these skins faces are random and only used as examples there are a endless amount of facial features to choose from but these are just examples* 
3.Ignoring them
ignoring can bt the best way to go sometimes and if you don't respond eventually they will give up and leave you alone
4.Telling them to Stop
One of the best things to do and one of the easiest *Why did I waste my time on writing these other examples XD eh still useful* Anyway Telling them to stop or to leave you alone is one of the best things you can do to prevent people from hitting on you/flirting though if they still do it tell a trusted friend/Moderator/Admin

But Anyway those are some tips and tricks to preventing Minecraft flirting and why it may happen,hopefully you found this useful and have a nice flirt free day :D*My goodless that last line sounded Lame*