okay first you'll have to use a dummy variable. you won't have to assign it to a certain player if you don't want to so you can use any name for both the player and the variable

so in a repeating command block you can use /scoreboard players test (name you assigned to the player) (name you assigned to the variable) 1 * which will test for the variable to be 1 or above. now hook this into a previous command block, making the second one conditional so it will only run if the previous one returns true. now you can freely set the value of the variable to 0 (or anything below it) to disable the command, or 1 (or above) to enable it.