Recently I've rejected a run (https://www.speedrun.com/super_mario_for_scratch_4/runs/yo0d0njy) because of use of speed up, however it doesn't necessarily state in the rules you can't do this so should there be a rule or should I accept it?

There should probably be a rule against it, as it is one of those things that would bring the run down to "Who has the fastest computer" rather than "Who has the most skill and/or dedication".

Added no speedup to the ruleset