Some points that might support your side of the argument might be that:

Sport within society is almost elevated to a religion.

Sport holds little moral worth within society (alike some religions)

Riots are caused every year by sports events (mainly soccer).

The current existence of sport is just a marketing tool to fuel profiteering in the corporate world.

Some rebuttals that might come up are:

Sport encourages good teamwork and ‘good sportsmanship’.

Sport holds much economically benefiting prestige that enhances the wealth of countries through tourism.

Partaking in sport encourages physical activity, preventing obesity.

There are two sides to every argument, so as well as preparing your own spiel, try to expose their statements as untrue or unjustified. Ask questions like, “Where’s your proof?” and “What specific evidence do you have to support your claim?” Try not to bury yourself by getting muddled and forgetting your original proposal, as your message will become unclear and the other team will then feel more confident and dominate the debate. Take some deep breaths, and act as though you know you’re right (even if you don’t, they might be fooled).

Hope you win, hope this helped.