Formal/ legal term for "enforcer"

As an unofficial position, in the UK, the person would be described as "the landlord's agent" if he receives some sort of reward for his action (e.g. money or a reduced rent, etc.) or, if he did not, it would be phrased "Mr Smith was acting as the landlord's agent" - hence he is an agent - but the term needs a description.

The official occupation is "bailiff" of which there are several types (See below), Bailiffs are not employed by the landlord - they work for a central organisation of some sort.

From the UK government website: https://www.gov.uk/your-rights-bailiffs

A bailiff (‘enforcement agent’) may visit your home if you do not pay your debts - such as Council Tax bills, parking fines, court fines and county court or family court judgments. […]

A bailiff may also visit your home for other reasons, for example to serve court documents or give notices and summons. There are different kinds of bailiffs, known as: ‘certificated enforcement agents’ ‘high court enforcement officers’, ‘county court and family court bailiffs’ and ‘civilian enforcement officers’,

Note that the term "bailiff" applies only in England and Wales. In Scotland, there are other official titles and there will be other titles in the jurisdictions of other countries and regions.

You should be specific as to which country you are referring.