How advertising works
The Old School RuneScape Wiki is hosted on Wikia, as are other wikis. Wikia is a wiki "farm". That means Wikia provides servers and server support for the wikis that provide content, but the wikis are administered independently of Wikia (with some oversight).
Wikia "publishes" wikis for free, but Wikia is a commercial enterprise. Wikia gets its revenue from the advertisements that are displayed on the wikis that it hosts. Old School RuneScape Wiki administrators are forbidden by Terms and Conditions from blocking advertisements. Wikia, however, does not create or provide the advertisements.
Wikia contracts with advertising exchanges to provide advertisements. An advertising exchange provides advertisements that appear on the wikis (and other sites) and pays Wikia a fee based on how many visitors view (and perhaps click) the advertisements on Wikia wikis. Other factors may affect how much an advertising exchange pays for an ad view, such as the geographical location of the visitor or the page history of the visitor. Advertising exchanges, however, do not typically create advertisements; that job belongs to advertising agencies, who create and place advertising on behalf of its clients.
On the wiki
Neither the Old School RuneScape Wiki nor Wikia have much control over what advertisements appear on any particular page. Wikia has a business relationship with its ad exchanges and can specify what types of ads should or should not be provided to it.
For example, many publishers are sensitive to pornographic content and malware distributors, therefore advertising exchanges make an attempt to filter such ads. However, advertisers are paying to have their ads seen, so usually filtering at the exchange takes place based on complaints raised by publishers about specific ads. How exchanges choose to deal with undesirable ads is entirely outside of Wikia's control.
All revenue from advertising on Wikia goes to Wikia Inc., which uses it to pay staff wages and server costs amongst other expenses. No revenue is ever received by administrators, editors, or visitors of the Old School RuneScape Wiki.
Bad advertisements on Wikia are those that do not comply with Wikia's advertising rules. Possible advertisements in this category can be advertisements that autoplay audio, popup advertisements that obscure the page, or advertisements that otherwise disrupt your viewing experience. If you find an advertisement that does not comply with these rules, you can report it at Special:Contact/bad-ad.
The easiest way to disable advertisements is simply to create a free Wikia account, as logged in users see reduced advertisements. An additional step, for those who wish to completely remove advertising, is to go to Special:Preferences and scroll down to the Appearance section, where you can find an option to disable all advertisements. It's as simple as that.
You stay logged in to Wikia for around 30 days between visits, although this period is reset when visiting Wikia again. Logging in to Wikia requires browser cookies, since login status is stored in a cookie.
If you do not wish to create an account, you can install an ad blocker, such as AdBlock, Adblock Plus, or uBlock Origin. You may install these at your own risk, and neither Wikia nor the Old School RuneScape Wiki accepts any responsibility for any issues that may arise from the use of such software. If you do use ad-blocking software and play the client in the browser, it is recommended to whitelist runescape.com to comply with RuneScape's advertisement blocking rules.