Zandronum

Is there a way to temporarily disable forward and backward movement, retaining the possibility to strafe, while making it possible to read forward and backward press through GetPlayerInput? Setting PROP_Frozen does not do what I want as it blocks strafing, too.
In the worst case I will use PROP_Frozen and block strafing.

if(player_movment==x || player_movment==y) spawn("mapspot",player_x,player_y,player_z+32,2,0);
else thingremove(2);

Idk what do you want, but this will make the player not moving on this keys xD

could morph the player to a class with forward and backward speed set to 0

forgot the property names, look 'em up, it should be on the Classes:Player page

No, morphing has alot of side effects - teleport fog, health reset, unability to switch weapon...

Using ACS to set two linedefs to impassable, trapping the player in invisible walls for the period of the event? That would be my way of doing it anyways!

The problem is, I cannot move the linedefs... You could just turn around just by one degree and the linedefs wouldn't work anymore.

Only thing I can think of is this.
Check if the player is frozen, if so, if the input for Strafing is being used (button pressed), then ThrustThing the actor in the proper direction.

and why dont you spawn some bridges on the forward and backward direction of the player? :S
It's weird but it'll work :p

Bridges won't work; they'll block the player's movement completely more often than not, due to hitboxes not rotating.

Adding velocity through ACS would work, but would also have a noticeable delay online.