This anecdote about Alkibiades is mentioned by Plutarch as an example of his grotesque disregard for common customs:

For instance, he once imprisoned the painter Agatharchos in his house until he had adorned it with paintings for him, and then dismissed his captive with a gift.

-- Plutarch, Life of Alkibiades 16.4

The practicalities of this treatment of poor Agatharchos would not have been too difficult. The typical Ancient Greek house consisted of a set of buildings around a walled courtyard, usually with only a single entrance facing the street. The very purpose of this design was to keep private space well separated from public eyes and public access; only those who were invited into the house could enter. Usually the women's quarters would be even further tucked away, restricted to inner chambers or to the upper storey of the house. These houses were built to create concentric circles of privacy. Already in the poems of Solon (early 6th century BC) we find a reference to people trying to hide "in the furthest room of the house" while evil forces "leap over the fence", showing that these spaces were designed to provide safety and keep unwanted people out. It would be just as easy to put its barriers to use in order to keep people in. Locks were well known and could be applied to either side of a door. And then, even if the painter could be heard from the street if he yelled, how would anyone be able to get him out?

For the same reason they built their houses like mini-fortresses with single entrances, the Greeks also generally did not add windows on the ground floor. Windows would typically have been small anyway, and may often have been too small for an adult man to wiggle through - but even if he could, he would risk falling to his death from the first floor.

But the final safeguard would be the fact that the house of a man as wealthy and prominent as Alkibiades would not simply be his own private bachelor pad. Even disregarding his family, which would presumably not be living there while the place was being redecorated, his house would have been inhabited by numerous slaves. House slaves (male and female) were a common feature of even modest households, and Alkibiades is likely to have had dozens at his disposal. It would not be that extraordinary to order a slave to watch every entrance to the house, or simply to station a slave by Agatharchos' side with orders not to allow him to leave until he was done. Prisons don't necessarily need bars and locks; a loyal household slave's keen eye would be more than enough coercion to keep the painter on site.