Form submit ondanks return false (ajax)

[code=default]
<form action="downloadCheck.php" onsubmit="return check();">
<fieldset>
 <div id="captchaimage"><a href="<?php echo $_SERVER['PHP_SELF']; ?>" onclick="refreshimg();" title="Click to refresh image"><img src="images/image.php?<?php echo time(); ?>" width="132" height="46" alt="Captcha image" /></a></div>
 <label for="captcha">Enter the characters as seen on the image above (case insensitive):</label>
 <input type="text" maxlength="6" name="captcha" id="captcha" onblur="" />
 <input type="submit" name="submit" id="submit" value="Check" />
</fieldset>
</form>

[code=default]
<form action="downloadCheck.php" onsubmit="return check();">
<fieldset>
 <div id="captchaimage"><a href="<?php echo $_SERVER['PHP_SELF']; ?>" onclick="refreshimg();" title="Click to refresh image"><img src="images/image.php?<?php echo time(); ?>" width="132" height="46" alt="Captcha image" /></a></div>
 <label for="captcha">Enter the characters as seen on the image above (case insensitive):</label>
 <input type="text" maxlength="6" name="captcha" id="captcha" onblur="" />
 <input type="submit" name="submit" id="submit" value="Check" />
</fieldset>
</form>

[code=default]
function check()
{
	var submission = document.getElementById('captcha').value;
	var url = 'process.php?captcha=' + submission;
	//process.php post: 1 of 0; 1= captcha oke, 0 captcha niet oke
	
	docheck(url, displaycheck);
}


function docheck(url, callback)
{
	http.open('GET', url, true);
	http.onreadystatechange = displaycheck;
	http.send(null);
}

function displaycheck()
{
	if(http.readyState == 4)
	{
		var showcheck = http.responseText;
		if(showcheck == '1')
		{
			return true; 
		}
		if(showcheck == '0')
		{

			document.getElementById('captcha').style.background = '#ffbcbc';
			
		    return false;
			
		}
		
	}
}

[code=default]
function check()
{
	var submission = document.getElementById('captcha').value;
	var url = 'process.php?captcha=' + submission;
	//process.php post: 1 of 0; 1= captcha oke, 0 captcha niet oke
 
	docheck(url, displaycheck);
}
 
 
function docheck(url, callback)
{
	http.open('GET', url, true);
	http.onreadystatechange = displaycheck;
	http.send(null);
}
 
function displaycheck()
{
	if(http.readyState == 4)
	{
		var showcheck = http.responseText;
		if(showcheck == '1')
		{
			return true; 
		}
		if(showcheck == '0')
		{
 
			document.getElementById('captcha').style.background = '#ffbcbc';
 
		    return false;
 
		}
 
	}
}

[code=default]
<form action="downloadCheck.php" onsubmit="return check(); return false;">

[code=default]
<form action="downloadCheck.php" onsubmit="return check(); return false;">