On Friday UUP MLA Robbie Butler posted a 'joint statement' from the DUP, UUP, SDLP and Alliance on Facebook.
It said that each party had signed up to a voluntary ban during their respective campaigns.
It was also posted by the DUP's Paul Givan and SDLP councillor Pat Catney.
However on Friday evening, spokespersons for the SDLP and the Alliance Party said no final agreement has yet been reached.
The so-called joint statement read: "As political parties in Lagan Valley, we recognise the desire from the public to minimise disruption during this election campaign and believe this is the right step to take.
"Engagement with the electorate will be focused on the door step."
According to the statement, the agreement is in place until 18:00 GMT on 1 March, at which point candidates may place a limited number of posters outside each polling station.
Current MLAs in the constituency, which takes in Lisburn and Banbridge, include Paul Givan, Edwin Poots and Brenda Hale of the DUP, Jenny Palmer and Robbie Butler of the UUP and the Alliance Party's Trevor Lunn.
The statement said that no posters will be placed on main or arterial roads unless directly outside a polling station, adding that it does not extend to billboard or mobile advertising.
"If this agreement is broken by one party, it is no longer valid," the statement added.
The spokesman appealed to other parties campaigning in Lagan Valley to follow the lead being given by the agreement.
A Northern Ireland-wide ban was proposed by Ulster Unionist chair Lord Empey last year.
At the time, the idea was dismissed by the DUP as "a stunt".
There is confusion over whether agreement has been reached by assembly candidates in Lagan Valley on banning election posters in the constituency ahead of the election in March.