I could be wrong, but I don't think you can completely disable the clock, because the engine (I'm assuming you're playing an engine?) has to have some instructions about when it should move.
But it's easy enough to get around the problem. If you just set the levels mode (menu - Levels -> Adjust) to "time per move", the engine will move within the specified time per move, but you can take as long as you wish. (The clock counts upward from zero in this mode.)
If you prefer something like a blitz or tournament mode that counts down to zero, you can adjust the clock at the beginning of the game and add some huge number of hours for yourself. (See the Arena Help files - Using Arena -> Adjusting the clocks.)
As the title says, I would like to disable the clock in Arena 3.0. I am a beginner and sometimes I take longer than the allotted time. Rather than constantly pausing the clock, or adding time as my time gets lower, I would like to simply turn the clock off and play an untimed game of chess. I can hide, pause, and adjust the clock, but I can't for the life of me figure out how to disable it completely. Is this impossible with this program?