Variant:
Melee attackers no longuer require to stand still to make a full-attack. A full attack can be distributed against as many opponents as the combattant wishes, with the only limit being his number of attacks and his move speed.
Example: A lvl 18 Fighter moves 10 feet, attacks 2 times a Troll, then moves 5 feet, lands an attack on a Minotaur and finally moves 5 feet and hits a lone standing archer.
Full-attacks can also be made after a move action against a single target.
What is stopping them now?
Any damage the combattant receives during his round stops this ability where he stands at the moment he took the damage. If I take the first example: A lvl 18 Fighter moves 10 feet, attacks 2 times the Troll, then moves to the Minotaur. But! When he tries to leave the threat zone of the Troll, he gets an attack of opportunity. If it hits, the Dash & Slash failed, only two options remain: Continue his full attack on the Troll (because it is the only target available), or abandon his remaining attacks to move to the archer in order to get closer (but lose his attack because he already attacked during his round).
Is there any way I can stop them of reaching my allies?
With Combat Manoeuvers. As an alternative attack, you can trip your foes, bull rush them, disarm them or even grapple them in an Attack of Opportunity, or in a readied action.