Unless there’s an agreement to the contrary, someone who commissions plans will usually have an implied licence to use them for the purpose for which they were commissioned (usually, to construct a building based on the plans). This will usually be the case whether or not the client has paid.

The best way to ensure a client can’t use plans until you’ve been paid is by ensuring your client agreement says something to this effect. We understand that the sample agreements available to members of the Australian Institute of Architects include a clause that makes the client's licence to use the plans conditional upon payment.