If another student has been treating you unfairly, or if you have a complaint to make against them, this is a two-stage process. Similarly to the university complaints procedure, we would always hope things can be resolved informally.
The first stage is an informal mediation session between the students involved, and the Advice and Wellbeing Coordinator at the Students' Union. This is an attempt to talk through issues maturely, and come up with a way to resolve things. This might be making some house rules if it's a housemate, or setting some boundaries with each other.
If this is not successful, and the informal process does not resolve the issues, you can make a complaint against the student under the Student Code of Conduct. Please click here to read through the code of conduct, and find out if the person in question has violated the code. If you feel you have grounds to make a report, you can do so by contacting Gateway for your campus, or your tutor. You can put together a report and pass it over to them. The university will investigate this, and depending on what has happened, some possible outcomes are a written warning, a fine of up to a maximum of £500, restrictions on access (e.g. to Students' Union bar or to University managed residential accommodation) or requirement to pay a reasonable sum by way of compensation. This is not an exhaustive list of outcomes, you can read more about this here.