If you want the easy choice, group your AT-ATs, and keep an eye on them. Be ready to click the stop order over and over when they attack something.
Otherwise, you might try editing your xml files (assuming EAW)
Open ground vehicles
ctrl+f 'AT_AT'
Till you hit a line that looks like this
<GroundVehicle Name="AT_AT_Walker">
Then ctrl+f 'chase
till you hit this block
<Idle_Chase_Range>0.0</Idle_Chase_Range>
<Guard_Chase_Range>200.0</Guard_Chase_Range>
<Attack_Move_Response_Range>150.0</Attack_Move_Response_Range>
<Targeting_Stickiness_Time_Threshold>5.0</Targeting_Stickiness_Time_Threshold>
<Targeting_Priority_Set>ATATAntiEverything</Targeting_Priority_Set>
<Autonomous_Move_Extension_Vs_Attacker>100.0</Autonomous_Move_Extension_Vs_Attacker>
and set both Guard_chase_range and Autonomous_Move_Extension_Vs_Attacker to 1.0 or 0.1
However, create a back up for your xml
before you do this, in case you want to replace it
Again this is for EAW and my best guess, hope it helps