Deny the installation of apps or add new accounts (kiosk mode) I have a tablet that I want to give to people to go on the internet, but I don't want them to mess with it (install/unistall apps, add new accounts, change relevant options, etc).

Is there a way to lock the device so that it can only be used to go on the internet?
SureLock promises this amongst others, see its entry on Google Play.

With Android's rapid development, this is however a rather quick moving target.

I suggest you search also for 'android kiosk mode' on Google or of course in the Play Store.

Besides this: There's already a question for that on stackoverflow:
Kiosk mode in Android