It is Standard Salesforce functionality.
By default the Salesforce allows you to add the name which starts with Account Name -. So that user can edit that name and use some other name (Generally people use Account Name-Opportunity Name).
Create a workflow rule so that opportunity name doesn't have “-" on lead conversion.
1.From Set Up | Create | Workflow | Workflow Rules | Click New Rule
2. Select the Object as Opportunity.
3. Enter Rule Name.
4.Then select the Rule Criteria as formula evaluates to true.
5.Click Save and Next.
6.Add Workflow Action to update New Field.
4.On Field Update Edit page Enter Name-Select Opportunity Name as the Field to Update.
5.Specify New Field Value as below:
SUBSTITUTE( Name , '-', '')
6.Activate the Workflow Rule.