Apparently, the website opens spam ads on some computers when accessed through Firefox or Chrome. I verified in the meantime, that our server was not infected. The most likely reason for this is that the computers which show these effects have been infected by malware.
Our server runs a supported version of Linux. All security updates are applied as soon as they are published. The website code is based on the Joomla framework, all inputs and forms are sanitized. And most important, the leagues in Itaewon strictly forbid any form of advertisement to generate income for the league or 3^rd parties.

Here are some hints on what to do for this and many other problems:

1. Do not use Windows XP anymore. This version of Windows is not supported with new security updates since beginning of April and is vulnerable to attacks from Spammers and Hackers.
2. Even if you use a newer Windows, you need to make sure that Windows (or any other operating system software such as iOS), firefox, chrome, safari, and all other software is always updated to the newest version. Exploits are regularly detected in software and are usually fixed within days.
3. Do not use an operating system or other software, for which you do not have a license (which you do not own).
4. Never follow links to potential spam websites. Do not open emails from unknown senders. Do not open email attachments unless you are sure the sender is trustworthy. Use browser extensions such as "NoScript" to prevent cross site scripting and other attacks.
5. The most likely culprit in your system is the browser you are using. Try to first clear the cache of the browser, including cookies and cached website pages. After that, immediately close the browser, reboot your computer and access the site again. If the problem persists, reinstall the browser.
6. If this also does not help, then you might have caught a virus in your computer. Scan your entire computer (all drives) with a good virus scanner. Note that you should do this by booting from a CD or USB drive (read only!), not when Windows is already running, as some viruses can hide themselves from the virus scanner.
7. Do not forget to scan all your USB drives and external hard drives for a virus, otherwise you get it back onto your system the next time you plug in the external drive.
8. Not all viruses are detected by all virus scanners. If you cannot solve the issue until now, you need to completely wipe your harddisk and reinstall Windows.

Here are some interesting links on the subject:

Troubleshoot Firefox issues caused by malware

Spam Prevention: Don't Let Spammers Find You!

Both Itaewon leagues, the IPL and the SIDL, are non profit organizations and as such, are not a business. Advertisement income would change this and would trigger tax liabilities, among many other obligations.