string with extra characters

I assume that you are using a simple SQL query such as

SqlCommand cmd = new SqlCommand("INSERT INTO tab (field1) VALUES (" + textBox1.Text + ")");

And the database complains?

Congratulations! This is leaving an opening known as "SQL Injection".

The addition of special characters causes the database to process them as part of a command. For example (DO NOT DO THIS) if you enter "hello);DROP TABLES tab" into your text box, the SQL would see:

"INSERT INTO tab (field1) VALUES (hello);DROP TABLES tab;)"

as a single command.
Because ';' is a statement terminator, SQl sees this as two commands: An INSERT, followed by a DROP TABLES. It inserts your name as "hello", and then deletes the tab table.

Don't do it this way. Use a parameterized query:

SqlCommand cmd = new SqlCommand("INSERT INTO tab (field1) VALUES (@NAME)");
cmd.AddWithValue("@NAME", textBox1.Text);

And you problem will disappear, instead of your tables...