My housemate doesn’t pay rent. Can I break into his room and sell his stuff to make the money that he owes?
No. That would be illegal, and leave you vulnerable to being prosecuted. Your choices for action are dependent on whether he is on the lease and whether you are on the lease. Start by asking him to pay his rent. This is by far the best solution.
If that doesn’t work, and you are not on the lease, you could notify the landlord of your situation and see if you are able to pay your rent separately. Chances are the landlord will not care who is not paying rent, and take you all to the tribunal, but as you are not on the lease you are not financially liable, and can find somewhere else to live. Keep receipts of you paying rent just in case.
If you are on the lease and he is not on the lease you could have him move out. If he has no written contract he has no rights, and you can ask him to move out immediately.
If you gave him a written contract, he is considered a sub-tenant, and you would be the head tenant, meaning he is covered by the Residential Tenancy Act. To get him to move out will require you to be compliant to the law, and you should seek legal advice (e.g., from the SRC Legal Service) to do this properly.
If you are on the lease and he is too, you are co-tenants. The easiest thing to do here is to leave the tenancy. Bear in mind that you are financially liable for whatever debts are incurred both as an individual, and as a group. Again seek legal advice to ensure that you get the best possible outcome.