The Lib Dem site was called "May & Co" - the same name as a Chelsea estate agent established in 1920.
The firm's boss, John Yianni, said he started getting abusive and "weird" phone calls when the campaign appeared on Twitter.
The Lib Dems said it was "an innocent mistake" and the site had been changed.
A party spokesman blamed an "over-enthusiastic volunteer" who had released an early version of the campaign which had not been checked.
He said the party had apologised to Mr Yianni.
The Lib Dem site is now called "Theresa May and Co" - but Mr Yianni said it was still close to the name of his company and he was concerned it was affecting his business.
"The damage has been done and it's probably going to get worse. You don't want to be politically biased in your business.
"People were saying: 'What are you doing getting into bed with them?'"
He said he had contacted Sir Vince Cable and the Lib Dem headquarters to ask for the campaign to be dropped.
"I was a Vince Cable supporter. I told him that," said Mr Yianni, who said he would not now be voting for the former business secretary.
He added: "This company has been going since the 20s. They should have just checked before they unleashed it.
"They have changed it into something slightly different thinking it's going to be OK but it's still the same."
Lib Dem candidate Sir Ed Davey earlier staged a protest against Conservative social care reforms, which the party has dubbed the "dementia tax," with "Theresa May & Co" placards.
Plans to make people receiving care at home liable for the full costs if they are worth at least Â£100,000 have proved controversial since they were announced in the Conservative manifesto.
Theresa May later said proposed changes to social care funding in England would include an "absolute limit" on the money people will have to pay.
She has repeatedly stressed that people will not have to sell their homes while they are alive to fund care.
Speaking at the protest, Sir Ed said there are other ways of funding social care than "setting up May & Co estate agents to come after your home".
A London estate agent has complained to the Liberal Democrats after they used the name of his company in a spoof website attacking Theresa May.