Modern mobile operating systems such as Android and Apple iOS allow apps to access various system resources, with or without explicit user permission. Running multiple concurrent apps is also commonly supported, although the OS generally maintains strict separation between apps.  However, an app can still get access to another app's private information, such as the user input, through numerous side-channels, mostly enabled by having access to permissioned or permission-less (sometimes even unrelated) resources, e.g., inferring keystroke and swipe gestures from a victim app via the accelerometer or gyroscope.  Current mobile OSes do not empower an app to defend itself from such implicit interference from other apps; few exceptions exist such as blocking screenshot captures in Android.  We propose a general mechanism for apps to defend themselves from any unwanted implicit or explicit interference from other concurrently running apps.  Our AppVeto solution enables an app to easily configure its requirements for a safe environment; a foreground app can request the OS to disallow access - i.e., to enable veto powers - to selected side-channel-prone resources to all other running apps for a certain (short) duration, e.g., no access to the accelerometer during password input.  In a sense, we enable a finer-grained access control policy than the current runtime permission model, and delegate the responsibility of the resource access decision (for vetoing) from users to app developers.  We implement AppVeto on Android using the Xposed framework, without changing Android APIs.  Furthermore, we show that AppVeto imposes negligible overhead, while being effective against several well-known side-channel attacks.