A lot of us have a sense of personal space while the rest sometimes don’t. The ones who do not have a perimeter around their person are the ones who tend to come into the space of others.
Just walk away when you can.
There is a chance that your colleague doesn’t know that he/she is doing it so tell ’em!
If you have been tolerating the person for long, there is a chance that the mere presence of them irritates you to the point of raised blood pressure. This makes blowing up very easy but don’t. You will unstable, immature and quite frankly, dumb.
Telling someone that your personal space is being entered might not be the easiest conversation to have especially if the person in question is your boss. Either way, be gentle and talk not shout.
Make your boundaries known.
Like I said before: the person might not know what he/she is doing wrong so making the mistakes known, along with your boundaries, should be done in a very nice way that will not affect the relationship afterwards.