I'm not sure I understand your question. It sounds like you're saying that someone else is using your hostel's wifi and, by doing so, is limiting your speed. Is it the wifi speed that's being limited, or are you complaining that the Internet speed is being used up or hogged by this other guy? In either case there's not much you can do, the bandwidth, both for the local wifi signal and for the Internet, are given out on a first-come, first-served basis. In other words, he who grabs it first gets it.
The best you can do regarding the local building's wifi signal is to get closer to the wifi router than the other guy, which will give you a stronger signal and more bandwidth. But still, when you pause in your use of it, he will be able to use it and depending on what he's doing, such as downloading a very large file, he may not give it back for a while.
Regarding the Internet, whoever gets it, gets it. If he's downloading large video files from somewhere on the Internet, he's going to hog most of your Internet bandwidth, there's not much you can do about it.