Well the body will accelerate as long as you are applying a force on it. So if u apply a force for forever it will accelerate forever but there is an interesting thing that happens.

Force applied for ever

You would notice that if u apply a force for an infinite amount of time then by $F=ma$ there should be acceleration for an infinite amount of time leading to a speed greater than light. Then is the time when you start to think that there is something wrong with this and indeed there is. Turns out that $F=ma$ isn't very accurate in this case but does this violate newton's second law? No, because newton's second law was never $F=ma$. This is a form it gets reduced to if the mass remains constant but we know that mass does increase with increase in velocity and considerably when speed reaches the speed of light.(The second law FΔt = p is still valid). So now we can see that as you reach the speed of light u need a larger force to have the same acceleration and at speeds really close to speed of light u will need an infinite amount of force to produce any acceleration in the body.

I know you have already had your answer but thought I should share this case which I really found interesting.

In classical mechanics, yes. Constant force leads to constant acceleration thus the particle will accelerate forever reaching infinite velocity. If the forces stops acting, then the body continues with constant speed.

However this would lead to a speed superior to that of light ($c$) which is prohibited by relativistic mechanics. So Newton's law must be modified.

Define the vector momentum as $textbf{p}=gamma m_0 textbf{v}$ where $m_0$ is the rest mass of the particle, $textbf{v}$ is the velocity and $gamma={1over sqrt{1+{textbf{v}cdottextbf{v}over c^2}}}$.

Then, as mentioned in another answers, we write the Newton's law as:

$$textbf{F}={dtextbf{p}over dt}$$ (which is actually the original form of Newton's law, but we usually deal with constant mass so that $F=ma$ is the same thing..).

Now assume the system is one dimensional (so no vectors!): Since velocity is also present in the $gamma$ factor: $$F={dpover dt}=gamma m_0 {dvover dt}+m_0vgamma^3{vover c^2}{dvover dt}$$ which we rewrite, using $a={dvover dt}$:

$$F=a m_0 gamma^3({v^2over c^2}+{1over gamma^2})=a m_0 gamma^3({v^2over c^2}+1-{v^2over c^2}) = a m_0 gamma^3$$

meaning $$a={dvover dt}={Fover m_0gamma^3}={Fover m_0}left(1-{v^2over c^2}right)^{3over 2}$$

This is our new equation. As you can see, at the beginning ($vll c$) it is the same as in classical mechanics. Then, as velocity increases, acceleration decreases, so that it can not reach the speed of light if not asymptotically.

One way to imagine this (though not completely right) is to rewrite this equation as

$$F=ma$$ as in classical mechanics but with a velocity-dependant mass $m=m_0gamma^3$ (longitudinal mass. There is an equivalent transverse mass used in highr dimensions when F is not parallel to v). Thus as speed increases, the mass becomes infinite and the acceleration goes to zero.

In conclusion, the solution to our equation is $$v(t)={Fover m_0}{toversqrt{1+{F^2over m_0^2}{t^2over c^2}}}$$ that, as you can see also from the plot (in which ${Fover m_0}=1$ and $c=1$) the speed $v(t)$ reaches $c$ for $trightarrowinfty$.

So speed does keep increasing but never reaches $c$! (It still is true that if the force stops acting the body stops accelerating).

Graph: x is time, y is speed.

I like to look at it this way. if in a perfect vacuum a force of say two pounds is applied to an object, the object will move in the direction of the force, with a force of two pounds indefinitely(unless acted upon by another force). If the object were to continue to accelerate as was suggested, then its original force of two pounds would grow because it is accelerating, and therefore would be creating force from nothing which is impossible.

For a real world example: If I accelerated a ping pong ball to a force of 15 pounds (by hitting it with a force of 15 pounds) it would be impossible for it to hit the wall with a 20 pound force,because it would be creating that extra 5 pound of force from nothing. The only way it could gain that extra force is if it received more force from something else (like gravity,wind,etc..).

So the object would maintain its original force and a uniform velocity and would not accelerate.

I hope this answer's your question.