Thank for your question.
He cannot prevent you from graduating, he can complain to the University and it's up to them what they do about it. It's unlikely in the extreme that they would prevent you from graduating, there was a test case in which a university attempted to block a student's graduation because of outstanding rent arrears of over £3, 000.00 from living in student rented accommodation. They were not allowed to block her graduation.
The landlord does have rights in pursuing the rent arrears against you (including interest). Your deposit should be lodged in a tenancy deposit scheme if you are occupying your property under an assured shorthold tenancy, which you will be unless you received a notice stating that it was not to be.
These schemes run dispute resolution procedure which you can initiate in the event you do not agree to the landlord's proposed deductions. You should have received details of the scheme in to which your deposit was lodged. If you did not then you should ask for them from your landlord/agent.
If you deposit was not lodged in a TDS then you could make an application for the return of the deposit and a fine payable by the landlord to you of three times the deposit amount.
Ask the landlord to provide you details of the TDS for your deposit, but cushion it by saying you will pay the last months rent immediately upon receiving (and checking) the details of it.
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