Yes they can.
Under ofgem rules if a tenant is directly responsible for paying the gas and/or electricity bills, they have the right to choose their own energy supplier and the landlord or letting agent should not unreasonably prevent this. If there is a default supplier clause in the tenancy agreement tenants are still entitled to switch supplier.
According to ofgem if letting agents and landlords include clauses in tenancy agreements forbidding tenants to switch suppliers, they are breaking the law.
We live in an age where consumers have plenty of energy suppliers to choose from and tools to help change suppliers easily – something the government actively encourage.
Further Reading: Switching supplier: quick guide for tenants here